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COMETS
BY
PROFESSOR OF ASTRONOMY AND DIRECTOR OF
FLOWER OBSERVATORY, UNIVERSITY
OF PENNSYLVANIA
LONDON
BAILLlfiRE, TINDALL AND COX
S Henrietta Street, Covent Garden, W.C. 2
1030
ALL RIGHTS RESERVED, 1930
PRINTED IN AMERICA
TO MY WIFE
MARY FRANCES FENDER OLIVIER
THIS BOOK IS AFFECTIONATELY
DEDICATED
PREFACE
The study of meteoric astronomy, es-
pecially in its theoretical aspects, gradually
drew the author's attention to the need of a
closer study of comets. This in turn led back
to the origins of all the minor bodies of the
Solar System and to similarities that exist be-
tween them. The appearance of the impor-
tant book by T. C. Chamberlin, The Two
Solar Families, also stimulated this interest.
Further no recent book on comets, which
covers the theoretical side, is available in the
English language. Consequently it seemed
that a book of moderate size, which would
cover briefly the history of the subject and
the present theories of origin, constitution,
changes, and dissolution of comets, would be
useful to the astronomer who does not special-
ize in the subject, as well as to the average
intelligent reader. It is the author's aim to
fill these requirements, leaving out entirely
any mathematical discussions.
This book docs not claim to cover the whole
field; only selected comets are described.
Also important work by some investigators is
viii PREFACE
perforce omitted. The book is intended as a
sequel to older books on the same subject,
such as for instance Chambers' The Story of
the Comets, rather than to replace them. It
is further intended as a direct sequel to the
author's book Meteors, as new information and
discoveries have permitted a considerable
extension of certain theories tentatively ad-
vanced in its closing chapters. Also the ties
between comets and meteors are so numerous
that some knowledge of the one is necessary to
understand the phenomena connected with
the other.
The author desires to express his sincere
appreciation to his colleague, Dr. S. G. Barton,
for kindly reading and correcting the manu-
script, and for helpful criticism in its prepa-
ration.
CHAS. P. OLIVIER.
Flower Observatory
University of Pennsylvania
Upper Darby, Pa.
February 27, 1930
CONTENTS
CHAPTER I
HISTORICAL REVIEW 1
CHAPTER II
GENERAL STATEMENTS 16
CHAPTER III
COMET GROUPS 37
CHAPTER IV
COMET FAMILIES 47
CHAPTER 1 *
THE TAILS OP COMETS i 58
CHAPTER \ T l
THE SPECTRA OP COMETS 79
CHAPTER vl, r l
II ALLEY' s COMET 94
CHAPTER VII\l
BIELA'S COMET 127
CHAPTER IX
SEVERAL INTERESTING COMETS 149
CHAPTER X
MOREHOUSE'S COMET 160
CHAPTER XI
PON8-WlNNECKE*8 COMET 168
CHAPTER XII
COMET 1910a 178
X CONTENTS
CHAPTER XIII
COMETS AND METEOR STREAMS .......................... 185
CHAPTER XIV
COLLISIONS OP COMETS WITH THE EARTH ................. 193
CHAPTER XV
OK UGINS OP COMETS ..................................... 207
ir CHAPTER XVI
CONCL BIONS ............................................. 220
APPENDIX .............................................. 233
INDEX .................................................. 241
CHAPTER I
HISTORICAL REVIEW
"When beggars die, there are no comets seen,
The Heavens themselves blaze forth the death of
Princes."
SHAKESPEARE, Julius Cupsar.
Records of comets have come down ,o us
from remote antiquity. It is evident t u it the
appearance of a brilliant p~^ unexpected
comet in the sky must havj . caught the atten-
tion of men long before any permanent record
of it could have been left ii'or posterity. But
from the centuries preceding the Christian
era on, numerous accounts of comets have
survived, as well as some t heories advanced
by Greek and Roman writers.
The very word comet is derived directly
from the Greek word Ko/x 177-775, the long-
haired one. Many books give the Latin
word coma, hair, as being the ancestor. Hence
the comet was "a hairy one." This name
came quite naturally from the long tails so
often accompanying the larger comets, and
which attracted attention as being unique in
form among other heavenly bodies, all of
which were considered spherical.
i
2 COMETS
For most older comets the Chinese left
more complete records than European nations,
and it is due largely to their observations
that approximate orbits can be computed for
.some comets which appeared over two thou-
sand years ago. The Japanese also left fewer
simiLar accounts. Comets are mentioned in
the extensive literature from the Euphrates
Valley \vhich has been slowly unearthed and
translated. As / specimen we quote that
referring to the comet of 1140 B.C. The
tablet gives an account of a campaign in
Elam and states "a comet arose whose body
was bright like ^'the day, while from its
luminous body a tail extended, like the sting
of a scorpion." 1
In 344 B.C. a comet is mentioned by Dio-
dorus Siculus as follows: "On the departure
of the expedition of Timoleon from Corinth
for Sicily the gods announced his success and
future greatness by an extraordinary prodigy.
A burning torch appeared in the heavens for
an entire night, and went before the fleet
to Sicily." 2
1 A. H. Sayce, Babylonian Inscriptions.
* Bibliotheca Historica, XVI, 11; also Plutarch, Timoleon.
HISTORICAL REVIEW 6
In 146 B.C. a comet appeared that is de-
scribed thus by Seneca: " After the death of
Demetrius king of Syria, .... there appeared
a comet as large as the Sun. Its disc was at
first red, and like fire, spreading sufficient
light to dissipate the darkness of night; after
a little while its size diminished, its brilliancy
became weakened, and at last it entirely
disappeared." 3
In 43 B.C., just after the assassination of
Julius Caesar, Suetonius thus describes the
comet then visible: "A hairy star was then
seen for seven days under the Great Bear.
.... It rose at about five in the evening, and
was very brilliant, and was seen in all parts
of the Earth. The common people supposed
that the star indicated the admission of the
soul of Julius Caesar into the ranks of the
immortal gods."
These quotations will give a general idea
of the effects produced upon the minds of
classical writers by the appearances of bright
comets. Humboldt states that : 4 "While these
bodies were considered by the 'Chaldeans of
Babylon/ by the greater part of the Pythag-
* Quaest. Nat., VII, 15.
4 Cosmos, 4, 558-60.
4 COMETS
orean school, and by Apollorinus Myndius,
as cosmical bodies reappearing at definite
periods in long planetary orbits, the powerful
anti-Pythagorean school of Aristotle and that
of Epigencs, controverted by Seneca, de-
clared comets to be productions of meteoro-
logical processes in our atmosphere. " Seneca
stated: "For I do not think comets are a
casual outburst of fire, but belong to the
eternal works of nature. For why should it
surprise us that comets, so rare a phenomenon,
should not yet be subject to the regulation of
any known laws and that their origins and
ends should be hid from us, who see them only
at immense intervals?"
The authority of Aristotle, which grew to
such tremendous proportions in the Middle
Ages, in this case played into the hands of the
popular beliefs, and the more ancient Chal-
dean theory, brought into Europe doubtless
by Pythagoras, was set aside. It is only just
to add that Aristotle gave reasons and argu-
ments, even if erroneous, for his conclusions
that comets were mere meteorological phe-
nomena.
It is quite evident that most persons, high
and low, considered comets as portents and
HISTORICAL REVIEW 5
prodigies, and their coming doubtless caused
universal fear. This has not entirely ceased
even in our own day, when their nature is
better understood, and their orbits can be
calculated. Literature is rich in allusions to
the awe inspired by comets. Shakespeare has
among other:
"Comets importing change of times and states,
Brandish your crystal tresses in the sky, . . . ."
Henry VI.
"As stars with trains of fire and dews of blood,
Disasters in the Sun. 1 '
Hamlet.
And then the famous quotation from
Milton:
"Satan stood
Unterrified, and like a comet burn'd
That fires the length of Ophiuchus huge
In th' Artick sky, and from its horrid hair
Shakes pestilence and war."
In Homer's Iliad, the helmet of Achilles is
described as being
"Like the red star, that from his flaming hair
Shakes down diseases, pestilence, and war."
In Thomson's Seasons we find the following
verses, which could have been written only
6 COMETS
after the fact was known that many comets
returned at regular intervals:
"Lo! from the dread immensity of space
Returning, with accelerated course,
The rushing comet to the sun descends:
And as he sinks below the shading earth,
With awful train projected o'er the heavens,
The guilty nations tremble."
All through history we find that the appear-
ances of comets were considered as prophecies
of deaths of kings, famines, wars, pestilences,
and. other ills to mankind. Halley's Comet,
to which a separate chapter will be devoted,
happened to come nearly at the times of many
important events in human history. It is not
too much to say that some of its appearances
actually had an influence upon contemporary
events, due to the mental reactions of those
who saw it. It should be needless to add that
its physical effects upon the Earth were in all
cases quite negligible.
It is strange that Ptolemy, the author of the
Almagest, the earliest great textbook on as-
tronomy which has come down to us, entirely
ignored the subject of comets. This is a great
pity, as we should like to know what his and
contemporary scientific opinions were. We
may safely conclude that in the Middle Ages,
HISTORICAL REVIEW 7
with those who studied and thought of such
matters at all, the conclusions of Aristotle
were generally accepted in this as in most other
scientific matters.
As late as the sixteenth century, the father
of French surgery, the eminent Ambroise
Par6, described the comet of 1528 as follows:
"This comet was so horrible, so frightful, and
it produced such great terror in the vulgar,
that some died of fear and others fell sick. It
appeared to be of excessive length, and was
of the color of blood. At the summit of it
was seen the figure of a bent arm, holding in
its hand a great sword, as if about to strike.
At the end of the point there were three
stars. On both sides of the rays of this comet
were seen a great number of axes, knives,
blood-colored swords, among which were a
great number of hideous human faces, with
beards and bristling hair." It is only just to
the memory of this great man to say that he
was but eleven years old in 1528, so his recol-
lections of its appearance were doubtless
affected by the wild stories he had been told
by others.
However, fifty-six years before, Johann
Miiller of Konigsberg, known as Regiomon-
8 COMETS
tanus, had made real observations of the
positions of the comet of 1472 on different
nights, showing that he at least considered it a
heavenly body, worthy of study. This comet
was visible three months, and its approximate
orbit has been computed on the basis of
his observations. Similar rough orbits, based
mostly on Chinese observations have been
computed for over forty comets which were
seen previous to 1472 A.D.
When the bright comet of 1577 appeared
there was at last a man who took up the
problem in a scientific way. This was the
famous Tycho Brahe. He made an elaborate
series of observations at his own observatory,
and was able to compare them with others
made at Prague. The lack of parallax in-
dicated by the two series at once showed that
the comet was certainly more distant than
the Moon. He endeavored to represent its
path by a circular orbit external to that of
Venus. At any rate this work fully dis-
proved the hypothesis that comets were ordi-
nary "meteors," as the word was then used.
His more famous pupil, Kepler, worked on the
comets of 1607 and 1618. He concluded that
comets traversed our system in rectilinear
HISTORICAL REVIEW 9
orbits. He considered them as numerous as
fishes in the sea. His further conclusions as
to their nature are so curious, coming from
a man who made such eminent discoveries as
to orbits, that they are worth quoting in part.
It shows how even such a man is influenced
by the beliefs and superstitions of those
about him. He says: "They are not eternal,
as Seneca imagined; they are formed of
celestial matter. This matter is not always
equally pure; it often collects as a kind of
filth, tarnishing the brightness of the Sun and
stars. It is necessary that the air should be
purified and discharge itself of this species of
filth, and this is effected by means of an animal
or vital faculty inherent in the substance of the
ether itself. This gross matter collects under
a spherical form: it receives and reflects the
light of the Sun, and is set in motion like a
star. The direct rays of the Sun strike upon
it, penetrate its substance, draw away with
them a portion of this matter, and issue
thence to form the track of light we call the
tail of the comet. This action of the solar
rays attenuates the particles which compose
the body of the comet. It drives them away;
it dissipates them. In this manner the comet
10 COMETS
is consumed by breathing out, so to speak,
its own tail."
We may remark that in all this mass of
conjecture and foolishness there arc certain
germs of truth, as we shall see later. Also,
before Kepler's time, at least two observers,
Jerome Fracastor (1483-1543) and Peter
Apian (1495-1552) had observed that comets'
tails always point away from the Sun.
That the orbits of comets are either much
elongated curves or parabolas was proved by
Dorffel of Saxony from his work on the comet
of 1680. This comet was visible eighteen
weeks. Borelli had, however, in 1665 already
expressed the idea that the orbit of the comet
appearing at the end of 1664 was parabolic.
Newton, however, proved that according to
the law of gravitation a body could move
around the Sun, situated at the curve's focus,
not only in an ellipse but also in a parabola
or hyperbola. Hallcy, the friend and con-
temporary of Newton, undertook the calcula-
tion of the orbits of twenty-four comets for
which there were, in his opinion, enough
observations. He was struck with the great
similarity of the elements of three of the
comets, and on the basis of this and the
HISTORICAL REVIEW 11
approximately equal intervals between their
appearances, he predicted the ^return of. the
comet, now known by his name. The trium-
phant vindication of his prediction finally put
cometary astronomy on a firm, scientific basis.
A full account of this will be given later in the
chapter on Halley's Comet.
Before leaving the historical side, a few
more interesting examples will be quoted.
The most striking example from the Old
Testament is found in the verse: "And David
lifted up his eyes, and saw the angel of the
Lord stand between the earth and the heaven,
having a drawn sword in his hand stretched
out over Jerusalem." 5 That a comet is here
meant is made the more certain by a later
statement by the great Jewish historian
Josephus, who in his accounts of the prodigies
which foretold the destruction of Jerusalem
by Titus says: "Thus there was a star re-
sembling a sword which stood over the city,
and a comet that continued a whole year." 6
Apropos of comets Bayle quotes a remark
said to have been made by Henry IV about
the astrologers who had been forewarning
* I CHRON., xxi, 16.
Josephus, Jewish War, 6, 5.
12 COMETS
him about his death, "They will be right some
day, and the public will remember the one
prediction that has come true, better than all
the rest that have proved false."
The same writer speaking of the pride of
men who thought their deaths so important
that God must send a comet to announce it,
"If we had a just idea of the universe we should
soon comprehend that the death or birth of a
prince is so insignificant a matter, compared to
the whole of nature, that it is not an event to
stir the heavens/'
The comet of 1680 inspired the famous
Madame de Sevign6 to write as follows, on
January 2, 1681, to the Comte de Bussy. 7
"We have here a comet it has the most
beautiful tail that could possibly be seen.
All the great personages are alarmed, and
firmly believe that heaven, occupied with their
loss, is giving intelligence of it by this comet.
It is said that Cardinal Mazarin being
despaired of by his physicians, his courtiers
considered it necessary to honor his last hours
by a prodigy and to tell him that a great
comet had appeared which filled them with
7 Guillemin, World of Comets, 30, 1877.
HISTORICAL REVIEW 13
alarm for him. He had strength enough to
laugh at them, and jestingly replied that the
comet did him too much honor. In truth
everyone should say the same, and human
pride does itself too much honor in believing
that when perforce we die it is a great event
among the stars."
An amusing proclamation was issued by the
Town Council of Baden, 8 Switzerland, in
honor of the comet visible in January, 1681,
whose "frightful long tail" extended over
80. Among other instructions "all were to
attend Mass and Sermon every Sunday and
Feast Day, and none was to leave church
before the sermon or remain away without
good reason : to abstain from excessive Carni-
val festivities and playing and dancing,
whether at weddings or other occasions.
Evening drinks were to be on a modest scale
and to finish at nine, after which all were to
go home quietly, without shouting in the
streets "
These quotations show that, even at the
end of the seventeenth century, it was ex-
ceptional when a person was found who did not
still believe in comets as portents of evil.
*Jr. B.A.A., 37, 241,1927.
14 COMETS
One of the most curious opinions is that
attributed to Bodin 9 by Bayle who says the
former believed "that comets are spirits, who
having lived innumerable ages on earth, and
being at last near death, celebrate their last
triumph or are brought again to the firmament
as shining stars."
The writer was told personally by a man,
who in 1910 was a small boy living in a town
in the interior of Asia Minor, that the report
of the coming passage of the Earth through
the tail of Halley's Comet created much
consternation in his neighborhood. Many
understood that their safety depended upon
getting into water up to their necks, to avoid
the harmful effects, hence as the day ap-
proached water barrels were filled everywhere
in anticipation. Apparently, however, they
were not used.
The belief in the baleful influences of
comets had not entirely died out up to very
recent years, even with persons of some in-
telligence. Among many examples that could
be quoted is that of the comet of 1861 which
was supposed to announce the beginning of the
9 Bayle's Dictionary.
HISTORICAL REVIEW 15
Civil War in America. For the benefit of
astrologers and similar charlatans, who work
on the superstitions of the ignorant, it is too
bad that a great comet did not appear in the
summer of 1914, just as the World War began!
But no bright comet was sufficiently obliging.
CHAPTER II
GENERAL STATEMENTS
"And there appeared another wonder in heaven; and
behold a great red dragon, having seven heads and ten
horns, and seven crowns upon his head. And his tail
drew the third part of the stars of heaven, and did cast
them to the earth."
Comets are of all bodies in the Universe, or
certainly in the Solar System, the most
difficult to explain both as to their origins and
behavior. Even the same comet at different
returns does not necessarily look like itself,
and very likely some surprising and unex-
pected changes may take place within a
period of a few days or sometimes in a few
hours.
The typical comet, however, has three dis-
tinct parts: the nucleus, the coma, and the
tail. In many cases, particularly with faint
comets, the nucleus and tail seem to be
missing. Hence, not unnaturally, the state-
ment is often made in textbooks and else-
where that the coma is the one part that must
be present, if the body is still to be classed as
a comet. The writer desires to take strong
exception to the implication of this definition,
16
GENERAL STATEMENTS 17
if not to the statement itself. It seems to
him certain that the coma could not exist
without a nucleus, which would serve to give
enough mass to hold the body together, at
least to whatever extent it is so held. That is,
of course, unless there are forces in action
within the coma of which we know nothing,
or the coma material is in an unknown physical
condition. While improbable, neither of
these possibilities can be ignored, and ac-
cepting them explains many things now far
from clear. Assuming, however, that the
coma obeys laws with which we are familiar,
then it would appear that the attraction
within such a mass of gas and small particles
would not be enough to hold it together,
without a nucleus, exposed as it is to tidal
actions on each visit to perihelion.
It seems necessary, therefore, to admit that
all comets must have nuclei, whether we see
them or not. This last is not a great difficulty,
because we have every reason to believe that
a comet's nucleus is not one solid body, but a
group of meteorites of assorted sizes. When
this group is large and condensed enough, it
will appear as one or more almost star-like
points. When it is too small to be visible,
18 COMETS
though still compact, or when its individual
members have scattered out somewhat, no
nucleus can be seen. In such a case we would
find a coma apparently lacking a nucleus.
Sometimes, probably the contrary takes
place, and the coma is lost, while the nucleus
remains. This is, we infer, the explanation
of such a body as asteriod No. 944. In this
case, the nucleus evidently was not scattered.
It may be explained on several hypotheses,
some much more probable than others: (i)
the nucleus is one solid mass, (ii) it consists of
several large masses so near together that
perturbations have no appreciable effect.
These are merely suggestions, there is no proof
that either of them is correct.
In the long run it seems that no comet can
permanently retain all the characteristics of a
typical comet. We may go further and say
that most comets will be wholly dissipated.
The order of loss is: first the material that
forms the tail, second the coma material
(after which the comet will usually be in-
visible), lastly the nuclear material. This
last when well distributed along the orbit
probably forms the usual type of annually
recurring meteor stream.
GENERAL STATEMENTS 19
So far only one comet has been spoken of as
though it always lived its life alone. But we
have comet groups, which consist of several
moving in almost the same orbit, but spaced
at irregular intervals along it. Did not all
these once form parts of a supcrcomet which
broke up into a number of smaller ones?
Then we see examples like Biela's Comet,
where a small comet divides into two before
our very eyes. Then we have complex ex-
amples like TempePs Comet, the Leonid
group which furnishes the great showers at
thirty-three-year intervals, and scattered
meteors all around the huge elliptical orbit.
This last fact is proved by at least a few
Leonids being met every November (sec p.
186). But comet, and the dense group, and
scattered Leonids all move in the same orbit.
Without gomgTurther into the matter here,
it seems that comets are aggregations which
in time lose their typical cometary character,
and whose units are scattered. As there
seems no way of explaining how a comet can
be rebuilt, this fact makes any theory of their
origin difficult, for it seems hard to admit that,
if of planetary age, they could have survived
so long. These questions will be dealt with
more fully in a later chapter.
20 COMETS
Comets move around the Sun in orbits
which are conic sections, the Sun being at one
focus. Three types of orbits occur, elliptical,
parabolic, and hyperbolic. A superficial di-
vision of published orbits would show about
one-quarter in the first division (100 ),
three-quarters in the second (300 =fc), and
about a score in the third. A smaller list 1
of 347 more accurate orbits gives in each
division mentioned the following numbers:
60, 275, and 12 respectively. The ellipse
is the only closed orbit of the three types,
therefore any body moving in either a true
parabola or a hyperbola would simply revolve
once about the Sun and then go off into the
depths of siuice. Nor would it ever return,
as it would be moving in an open curve. On
the contrary, a body in an elliptical orbit will
move around the Sun in a definite period,
returning at regular intervals. 2
Are then the vast majority of comet orbits,
classed as parabolic, really parabolas? And if
1 W. W. Campbell, Adolph Stahl Lectures on Astronomy, 33,
1919.
2 Stromgren, Vierteljahrsschrift d. Astr. Gesellschaft, IV,
1910. Also, Fa yet, Recherches C oncer nant lea EccenlriciUs des
Cometes, 1906.
GENERAL STATEMENTS 21
not, why are they so called? There is every
reason to believe they are not indeed, due to
planetary perturbations, a comet cannot move
in a true parabola. For if the comet started
toward the Sun in such a curve, the least
increase of its motion due to any planet would
make the orbit hyperbolic, the least decrease
would make it elliptical. One of these con-
tingencies must always occur.
But we only see a small part of the total
orbit, that which lies near perihelion. So
when the period is very long, that portion of
the orbit, though the latter is a true ellipse,
cannot be distinguished from a parabola.
This is due to the inherent imperfection of any
observations made by human beings, and
besides a comet is a more difficult object to
observe accurately than a star. Therefore, a
parabola fits the observations as well as any
other curve, and is far easier to compute than
an ellipse. So we say that they move in
parabolas though it is understood they ac-
tually do not, only our observations are not
accurate enough in such cases to give the
elements of the very elongated ellipse in
which the comet really moves.
As to the few hyperbolic orbits, in nearly
22
COMETS
all the cases which are capable of complete
investigation, it has been found that planetary
TABLE i
NUMBER OF RECORDED COMETS
TIME INTERVAL
NUM
HER OF COM!
STS
H
c
N-E
To 1 B.C
63
81
1 to 100 A.D
21
22
22
100 to 200 A.D
18
22
23
200 to 300 A.D
36
39
44
300 to 400 A.D
21
22
27
400 to 600 A.D
19
19
16
500 to 600 A.D
24
26
25
600 to 700 AD
21
33
22
700 to 800 A.D
13
17
16
800 to 900 A.D
31
41
42
900 to 1000 A.D
20
30
26
1000 to 1100 A.D
28
38
36
1100 to 1200 A.D
22
31
26
1200 to 1300 A.D.
26
30
26
1300 to 1400 A.D
31
34
29
1400 to 1500 A.D
36
46
27
1600 to 1600 A.D.
38
40
31
1600 to 1700 AD
27
36
12(b)
1700 to 1800 A.D
96
73
36(b)
1800 to 1900 A.D
(284) (a)
336
Total
1056
Note: In the above table the authorities are as follows :
H = Herz in Handwdrterbuch der Astr., Vol. II, 53, 1898.
C - Chambers in The Story of the Comets, 244, 1910.
N-E = Newcomb-Engelmann's Astronomic, 440, 1922.
(a) = To 1895 inclusive only.
(b) = Included only naked eye comets.
GENERAL STATEMENTS 23
perturbations were responsible for the slightly
increased velocity. 3 Allowing for these, the
orbit is seen to be the usual parabola or very
long ellipse. 4
While all the large planets, and most of the
asteroids, have orbits but little inclined to the
plane of the ecliptic and they all move with
direct, i.e., counter-clockwise motion, this is
not true for comets. Omitting the short
period comets of Jupiter's family, the rest are
about equally divided between direct and
retrograde motion, while their orbits have all
possible inclinations.
To make the above statements clearer, a
few statistics will be given (see table 1).
Holetschek gives the perihelion distances of
409 comets up to 1917 as follows:
0.00 to 0.49 92
0.50 to 0.99 173
1.00 to 1.49 94
> 1.50 JO
409
3 Pub. A.S.P., 23, 124, 1911. Also, Newcomb-Engelmann,
Astr. Pop., 441, 1922.
4 The recently published orbit of Comet 1926 VII, according
to Crommclin, has an eccentricity of 1.008658. This excess
over unity (i.e., the parabola) is too great to be attributed to
errors of observation.
24 COMETS
To show that with better observations few
comets have parabolic orbits: 5
Before 1755 99 per cent were computed as
parabolic
1756 to 1845 74 per cent were computed as
parabolic
1846 to 1895 54 per cent were computed as
parabolic
Comets visible
1 to 99 days 68 per cent were computed as
parabolic
100 to 239 days 55 per cent were computed as
parabolic
240 to 511 days 13 per cent were computed as
parabolic
COMET MASSES
An excellent example of the changes
wrought on a comet's orbit by a planet, with
no appreciable reciprocal action, which fact
proves the very small mass of the comet, is
furnished by Lexell's Comet of 1770. Exact
6 L. Rod6s, S. J., El Firmamento, 303, 1927.
Those specially interested in the statistics of the elements
of the orbits of comets should consult "Comet Catalogue" by
A. C. D. Crommelin in Memoirs of the British Astronomical
Association, Vol. 26, Part 2. This contains 561 orbits, but in
some cases several refer to the same comet, as the catalogue
aims to give the fullest information for each return of periodic
comets. This publication is a sequel to J. G. Galle's Come-
tenbahnen, 1894, which contains orbits of 411 comets up to
1894.
GENERAL STATEMENTS 25
calculation showed that it was very near
Jupiter in 1767 and by the attraction of the
planet had its orbit changed to one of small
eccentricity and 5-2-year period. Again in
1779 it came very near Jupiter, in fact passing
actually through its satellite system. But the
periods of the satellites were not at all changed,
so far as observations could show. This meant
that the comet had a mass less than -^- Q that
of the Earth. This time its eccentricity wasT
immensely increased, as well as its period, so
that the comet has never been seen since.
Incidentally in 1770 this same comet passed
near the Earth and had its period shortened by
2% days as a consequence. It came within
about 1 million miles; but the length of the
year did not vary one second. Russell states
that had the comet's mass been j--^ that
of the Earth a change of at least this amount
in the length of the year would have resulted. 6
Similarly Brooks's Comet in 1886 by nearing
Jupiter too closely had its period changed
from 29 to 7 years. In 1889 the comet was
observed by Barnard at Lick Observatory to
be double, the two parts separating at a rate
6 Astronomy, 430, 1926
26 COMETS
that would indicate that the disruption had
occurred three years earlier, when it was so
near Jupiter. 7
But as the Earth weights 6.59 x 10 21 tons,
should a comet weight only one-millionth as
much, it would weigh 6.59 x 10 15 tons. If one-
millionth of this still it would weigh 6.59 x 10*
tons a very tremendous weight according to
terrestrial standards. Yet neither of these
hypothetical masses would produce observ-
able perturbations. All we can honestly say,
therefore, is that comets' masses are too small
to produce observable effects by which they
could be evaluated. But in no case can it be
asserted that their masses are not large com-
pared with that of terrestrial objects. The
statement made by a prominent astronomer
about a century ago "that a comet could be
packed in a portmanteau," and one met
with in certain books that collision with a
comet would produce no harmful results to
the Earth, are both so absurd that they do
not deserve serious discussion (see p. 193).
Sometimes it is found that perturbations
suffered by a comet as it passes a planet on one
7 Young, General Astr., 454, 1904.
GENERAL STATEMENTS 27
occasion may later be reversed. An excellent
case 8 in point is furnished by Wolf's Comet.
Briefly, in 1875 this comet was so perturbed
by a close approach to Jupiter that the in-
clination of its orbit was changed from 29.4
to 27.5 and its period from 8.53 to 6.82 years.
Until 1922 it continued in an orbit essentially
the same, but between July and December of
that year it was again very near Jupiter. The
result was that on leaving its sphere of
activity the comet's orbit had an inclination
of 29.1 and a period of 8.28 years, in other
words its new orbit was almost the same as
that followed in 1875.
The coma of a comet, paradoxically, always
contracts on nearing the Sun, and expands as
it recedes. The tail, on the contrary, develops
in length the closer the comet approaches the
Sun.
The condition of the coma of a comet, as the
body nears perihelion, has long been a difficult
point to explain. Any theory, which has the
least probability, therefore, would be of a
certain value. In view of this fact, some
ideas bearing upon this question will now be
8 A. J., 34, 133, 1922.
28 COMETS
outlined, though it is at once added that they
are applicable directly to comets which have
tails. If comets of short period, without tails,
be included in the discussion, modification
will be necessary.
To take a concrete case for illustration,
Halley's Comet at its last return will be
Fio. 1
chosen. It was discovered at 3 units dis-
tance from the Sun (position A) being then
14,000 miles in diameter. At 2 units dis-
tance (position B) it had grown to 220,000
miles. But at perihelion, when at distance 0.6
units (position C), it had contracted to a
diameter of 120,000 miles. On the return
journey back from perihelion, it first ex-
GENERAL STATEMENTS 29
pandcd very considerably in fact to even
larger dimensions than it had as it ap-
proached and finally disappeared at 4 units
distance (position D), when it was 30,000
miles in diameter.
Following the usual theory, the comet at A
is supposed to have just begun the develop-
ment of its coma by gas oozing from the solid
meteorites of the nucleus, under the warming
influence of the Sun. This gas carries fine
dust with it. The gas and dust diffuse out-
ward in all directions, and so the coma ex-
pands. The dust reflects the sunlight, but the
gases absorbing solar energy begin them-
selves to shine. As the activity increases, the
finer particles are driven backward to form the
tail, which latter is lacking at first and is most
developed at or just after perihelion passage.
So far nothing has been added to what is gen-
erally accepted, but here we will use another
viewpoint. Let the position B be that at
which the coma has its maximum diameter,
the tail as yet having scarcely begun to form,
or at any rate being very small. Then this
may be considered that critical point at
which a balance is reached, giving a maximum
diameter to the coma which is subject simul-
30 COMETS
taneously to the inherent forces tending to
enlarge it and the depleting effects of the
radiation pressure from the Sun. Up to B the
former are the greater, afterwards the latter.
In other words while indeed the further ap-
proach to the Sun may cause increased ac-
tivity in the nucleus, yet this extra material
is driven away more rapidly as the radiation
pressure becomes more effective.
Thus from B to C the finer material, which
must always have formed the outer shells of
the coma, is being driven backwards into the
tail faster than it can be supplied from within.
Hence the coma actually would contract until
perihelion is reached. Meantime the tail
would increase in length and content. Both of
these phenomena are generally observed in
bright comets.
On the return journey, we would find the
inverse order, only the maximum develop-
ment would come after perihelion, due to the
lag in the heating effects of the Sun. Just
as our hottest days of summer come after
and not at the summer solstice. As a further
consequence, we would expect at any given
distance from the Sun a greater activity on the
return journey, for the comet is still using the
GENERAL STATEMENTS 31
stored up energy just gained by its close
approach. Also somewhat larger particles
would now have been pried loose from the
meteorites in the nucleus by the accumulated
energy, and so a larger coma would be ex-
pected. Because, at a given distance from
the Sun, larger particles would now form the
outer shells of the coma. Hence there would
be less susceptibility to radiation pressure and
a larger coma would result. As the comet
went still farther out, the activity of the
nucleus would decrease correspondingly, until
the comet would be too faint to be longer
seen. Nevertheless we would expect a some-
what larger size up to the very last at cor-
responding distances, than when it was on its
inward journey.
The above theory appears to fit fairly well
the case of the average bright comet. If it
be applied to one like Encke's Comet, here
indeed is found the contraction of the coma,
but practically no tail. Also these comets are
absolutely fainter than the class first con-
sidered. With more hesitation it is suggested
that the coma is depleted in exactly the same
way as before, when the body nears perihelion,
but the sum total of what is driven out is not
32 COMETS
sufficient to form a visible tail, at least in most
cases. If this general idea can be accepted as
accounting for the contraction of the coma, at
least we will be relieved from postulating un-
known causes for this so far unexplained
phenomenon.
The question of jets and other similar ex-
plosive changes in nuclei has been deliberately
ignored here, because such phenomena are
apparently additional to those by which the
average coma is normally developed. The
development of envelopes would seem to
occupy a middle position between slow, nor-
mal development of the coma and violent
changes. In their case it would seem that
the forces had to accumulate a certain amount
of energy in the nucleus, and then a greater
amount of material was emitted at one time
for their formation.
As to the transparency of the heads of
comets, the following statements will be of
interest. Struve 9 speaking of Halley's Comet
on September 24, 1835 says: "At Dorpatthe
star was in conjunction only 2.2" from the
brightest part of the comet. The star re-
9 Cosmos, 1, 89.
GENERAL STATEMENTS 33
mained continually visible, and its light was
not perceptibly diminished whilst the nucleus
of the comet seemed to be almost extinguished
before the radiance of the small star of the
ninth or tenth magnitude."
Bessel 10 on September 29, 1835, saw a 10
magnitude star at 7.8" from the nucleus of
Halley's Comet, therefore, in very dense
nebulous matter. The star's light experi-
enced no deflection from refraction.
Van Biesbroeck, 11 at Yerkes Observatory, on
June 22, 1927, took a photograph of a field of
stars, with the 40-inch refractor, right through
the head of Pons-Winnecke's Comet. He
found 73 stars of about 12 and 13 magnitude
on the plate, whose area was 35' by 35'. The
same region was again photographed when the
comet had passed on. No indication of shifting
of the stars could be detected, due to their
light having passed through the coma, which
was at least 100,000 miles thick. Any shift as
great as O.I 7 ' could certainly, and one of 0.05"
could probably have been found.
The ancients 12 were struck by the phenom-
10 Astr. Nach., 288, 303, 1836.
11 Cosmos, 1, 90.
12 Pop. Astr., 35, 499, 1927.
34 COMETS
enon that it was possible to see through
comets as through a flame. The earliest
evidence is that of Democritus. Seneca says:
"We may see stars through a comet as through
a cloud, but we can only see through the rays
of the tail, and not through the body of the
comet itself."
Comets are discovered in a variety of ways,
some by accident, others by persons who
make a business of comet hunting. At present,
some comets are picked up on developed plates
when the observer had been photographing a
region of the sky for a totally different pur-
pose. Some are first seen by persons not at
all interested in astronomy, as happened in
the case of a bright comet not many years
ago, discovered by railroad workers in South
Africa.
Certain men stand out most prominently
as being particularly successful in finding
comets. Pons is credited with no less than
37 between 1801 and 1827; he was then door-
keeper at the Marseilles Observatory. Two
other Frenchmen during the late eighteenth
century were also particularly lucky, Messier
and Montaigne. The following story told of
the former will illustrate what a passion comet
GENERAL STATEMENTS 35
hunting had become with him. It is a quota-
tion from La Harpe: 13 "He is a very worthy
man, with the simplicity of a baby. Some
years ago he lost his wife, and his attention to
her prevented him from discovering a comet
he was then on the search for, and which
Montaigne of Limoges got away from him.
He was in despair. When he was condoled
with on the loss he had met, he replied, with
his head full of the comet, 'Oh, dear, to think
that when I had discovered twelve, this Mon-
taigne should have got my thirteenth/ and
his eyes filled with tears, till, remembering
what it was he ought to be weeping for, he
moaned, 'Oh, my poor wife/ but went on
crying for his comet."
During the past fifty years, Brooks, Barn-
ard, Pcrrino and Swift in America, and Gia-
cobini of Nice have discovered between 10
and 20 comets each. The discovery of 5
comets by Barnard, while he was at Nash-
ville, Tennessee, and just starting his great
astronomical career, helped him through a
critical period of his life. In those days H. H.
Warner offered a prize of $200 to a person
13 Abbot, The Earth and The Stars, 87, 1926.
36 COMETS
discovering a new comet a reward alas no
longer available! Gaining these five prizes
largely enabled Barnard to pay for his home,
which was afterwards known as "Comet
House" to commemorate this fact.
The writer frequently has heard a story
about a prominent astronomer which is worth
telling, though he does not personally vouch
for its accuracy. This man, who was an
assiduous observer, was measuring a certain
comet every morning just before dawn. As
usual the comet was measured on a certain
date, but on reduction the place was 1 off
from where it should have been. Next
morning a search showed two comets! The
observer had discovered an entirely new
comet because he had made an prror of 1 in
setting his declination circle the morning
before. In view of this, it would be hard to
prove that errors, even in scientific work,
never have any value!
CHAPTER III
COMET GROUPS
By definition a comet group is composed of
at least two comets, which, while obviously
not the same body, yet move in practically
the same orbit. The most famous of such
groups is composed of the great comets 1668,
1843, 1880, 1882, and 1887. As no one of the
last four, no matter how much allowance is
made for errors of observation, could be re-
turns of the same body, that possibility is at
once ruled out. The elements of these five
comets are given in table 2.
A glance at these figures (table 2) shows at
once that all these comets passed within from
78,000 to 288,000 miles of the Sun's surface,
They therefore passed directly through the
solar corona. This means that the focus is
almost at the very end of the ellipse, which is
therefore excessively narrow and long. In-
deed the comets came toward the Sun and
retired therefrom in almost straight lines.
Coming so near, they made the turn through
-18Q_of heliocentric longitude in a few hours.
However, when so close in they were subjected
37
38
COMETS
to most terrific heat and radiation pressure,
which at least for the 1882 comet split the
nucleus and in all cases caused the develop-
ment of extremely long, straight tails.
The comet of 1843 was not discovered until
the day after perihelion passage which was
later computed to have been on February 27,
1843, at 10 h 29 m Paris M.T. It was first seen
TABLE 2
ELEMENTS OF THE GREAT COMETS OP 1668, 1843, 1880, 1882,
AND 1887
COMET
q.
e
t
n
IT
PERIOD
1668
0047
1
36
357
277
?
1843
0.0055
99989
36
1
279
1880 I
0059
99947
37
356
278
1882 III
0082
99993
38
346
276
1887 I
0054
1
43
337
274
?
in full sunshine on February 28, l-J- east of the
Sun's center and with a tail 4 to 5 long. In
a day's time it described 292 of its orbit,
leaving only 68 for the hundreds of years
before its return. At perihelion its velocity
was 342 miles /sec.
The following observations of its appear-
ance are quoted. 1 "On March 6 about seven
* Silliman's Jr., I, 44, 414, 1843.
COMET GROUPS 39
o'clock it presented a long, narrow brilliant
beam slightly convex upward, the lower end
being apparently below the horizon (it ex-
tended above horizon 30) ; the breadth was
about 2 at upper extremity and less than 1
where it was lost in vapors of horizon." "On
March 7 it extended 43. Its breadth near
the horizon was less than 1, and generally in-
creased towards the upper extremity, where
it may have been equal to 2. The curva-
ture of the train upwards, although very
noticeable, scarcely exceeded 2. The light
was nearly uniform." "On March 17 the
comet shone with great brilliance; the curva-
ture of train was less; its length was 34.
It was last seen here (New Haven, Conn.) by
the naked eye on April 3, when (due to Moon)
it was barely discernible."
A ship's captain, near the equator, saw its
tail 69 long on March 4. General Ewart,
who was on shipboard near St. Helena, says:
"It was a grand and wonderful sight, for the
comet now extended the extraordinary dis-
tance of one-third of the heavens, the nucleus
being, perhaps, about the size of the planet
Venus." 2
* Chambers, 142.
40 COMETS
The comet of 1880 (I), was discovered on
February 1 at several places in the Southern
Hemisphere. It passed perihelion four days
before, and was visible about three weeks in
all, being unfavorably placed for observation.
Gould at Cordoba stated that at no time was
there a nucleus, the head appearing cloud-like
and filmy, and elongated in the direction of
the tail, which it did not much surpass in
brightness. A tail 40 long, and from 1J to
2 wide, was seen on February 7, but its
brightness was not superior to that of the
Milky Way in Taurus. On February 4 it had
a coma 2' to 3' in diameter.
Comet 1882 (III) was one of the finest on
record, and, as it was observed for nine
months, the orbit was excellently determine^
and all its phenomena studied in great detail-
This comet was seen first as a naked-eye object
on September 3, in New Zealand. In four
days it had become so conspicuous that it was
visible at noonday near the Sun on September
16. It transited the Sun on September 17.
The comet was traced right up to the Sun's
limb by Elkin and Finlay at the Cape of Good
Hope, where it disappeared as suddenly and
effectively as a star occulted by the Moon.
COMET GROUPS 41
The nucleus was then 5" in diameter, but no
trace of it could be seen on the solar disc. 3 It
was visible for two days afterwards in full
sunlight. Its perihelion occurred on Sep-
tember 17, when later on the same day it
could be seen within 2 of the Sun. Because
of its small perihelion distance, it, like the rest
of this group, passed through the corona.
The changes in the head of this comet were
so great and remarkable that Young's words 4
will be quoted, he having himself been a most
able observer: "When the comet first became
telescopically observable in the morning sky
it presented a very nearly normal appearance.
The nucleus was sensibly circular, and there
were a number of clearly developed concentric
envelopes in the head; the dark, shadow-like
stripe behind the nucleus was also well marked.
In a few days the nucleus became elongated
and finally stretched out into a lengthened,
luminous streak some 50,000 miles in extent,
upon which there were six or eight star-like
knots of condensation. The largest and
brightest of these knots was the third from
3 Certain observations, claimed to have been made of this
transit, have been generally considered most doubtful.
4 General Astr., 451MK), 1904.
42 COMETS
the forward end of the line, and was some 5,000
miles in diameter. This 'string of pearls'
continued to lengthen as long as the comet
was visible, until at last the length exceeded
100,000 miles."
The changes began to be noted about Octo-
ber, and were seen by many observers, so
there is no doubt of their reality. A further
proof of disruption was one or more masses or
shreds of cometary matter, perhaps 3 distant,
that were seen, by at least three astronomers
in different places and on different nights, to
be accompanying the main comet. The one
noted by Schmidt at Athens, on October 9,
was seen on several subsequent days. Its or-
bit proved to be quite similar to that of the
main comet.
Russell's remarks 5 will help us to understand
why abnormal behavior might have been
expected after such a near perihelion passage:
"The great comet of 1882 passed, at peri-
helion, through a region where it was about
3000. Its nucleus, which survived the pas-
sage though hr^aking^jnto four parts, must
have been compose^ ^)f masses of considerable
Astroph. Jr., 69, 54, 1929.
COMET GROUPS 43
size. As this comet is undoubtedly periodic,
it must have been stripped of all finer material
at earlier returns, and is probably far from
typical of other comets." It seems that we
should add other comets " whose perihelia are
not equally near the Sun."
Due to the relative position of Earth and
the comet, its tail, though quite 100 million
miles in actual length, never subtended an
angle of more than 35. It fell under Br&li-
khine's second or hydrocarbon type (Young).
Tempel suggested that the tail was tubular in
form, while a drawing by Hopkins on Novem-
ber 14 shows the wider end bifurcated. Hold-
en's drawings show three nuclei on October
13 and 17, while Cruls saw two on October 15.
The latter suggested that these nuclei, 7 and
8 magnitude respectively, each had a tail,
and that the curious appearance of the one
tail was due to there being really two super-
imposed.
Besides the phenomena mentioned, the
comet also had a sheath of light that enveloped
the head and extended 3 or 4 in front of it.
This was faint and straight-edged.
As for the period, before the nucleus di-
vided, it was calculated to be 800 to 1000
44 COMETS
years. Kreutz calculated the periods of each
of the four nuclei into which the original
broke up. These turned out to be 664, 769,
875 and 959 years. We will probably see four
comets of lesser magnitude return about the
years 2546, 2651, 2757, and 2841 A.D., in place
of the one great body as in 1882.
As for its spectra, when very near the Sun
the sodium D lines showed very bright, also
the E and some other iron lines were reported
bright, as well as five unidentified lines in the
red. On September 18, the radial velocity of
the comet was determined at both Dun Edit
and Nice, the results agreeing well with the
calculated velocity. As the comet receded
the bright lines faded, the D lines lasting
longest, and the carbon bands came into greater
prominence.
It should, however, be stated that the iden-
tification of the iron lines mentioned has more
recently been gravely questioned. Indeed
before the days of photography the identifica-
tion of lines by visual methods was most diffi-
cult, and errors were to be expected.
Comet 1882 (III), for which there were so
many observations, was used by Hufnagel 6 to
Bui. Astr. Soc. de France, 34, 300, 1920.
COMET GROUPS 45
see if an orbit calculated on Newton's Law,
or that on Einstein's theory, would best fit
the facts. But the observations fitted one
as well as the other; hence no choice could be
made. As the comet came within 0.67 radius
of the Sun's surface, and was moving at 478
km /sec, he concluded that there was no trace
of a resisting medium and that the zodiacal
light could not be made up of ellipsoids of
density increasing as the Sun is approached.
Another small comet that was discovered
within less than 1 of the Sun on a plate taken
during the total solar eclipse of May 17, 1882,
very probably belonged to this same group.
The comet of 1887 belonging to this group
was discovered by Thome at Cordoba, on
January 18. It was observed five days later
at Melbourne. The comet was readily visible
to the eye in the strong twilight, the tail being
long, straight, and narrow.
Part of the description by Tebbutt will be
quoted: "Its position was on the southwest
horizon (January 20), but nothing could be
seen of the nucleus (January 28), the tail
could be faintly seen extending over many
degrees, and I noticed at its lower or brighter
extremity a star, which I took for the nucleus.
46 COMETS
On pointing the 4J-inch equatorial I saw the
supposed nucleus was really a fixed star.
Although the lower extremity of the comet-
ary ray or beam was certainly in the imme-
diate neighborhood of this star I could not
find the slightest condensation. The tail was
straight." (January 30) "My search for a
nucleus or even the slightest condensation as
a point of observation was again quite un-
successful, (February 1) not the slightest
trace of the tail could be seen owing to the
brilliancy of the Moon." 7
In fact never did this strange object have
any condensation in the head, which made its
orbit depend upon uncertain observations.
It passed perihelion January 11, and the length
of its tail shortly after discovery was nearly
40. We might well wonder whether a simi-
lar object, which did not approach nearer than
0.5 unit of the Sun, would ever be visible at
aU?
Observatory, 10, 66-7, 1887.
CHAPTER IV
COMET FAMILIES
In practically all textbooks written ten or
more years ago, the statement is made that
the four major planets of our Solar System
^ --.-. . .. A. ... - . ~ _,__.._ . . . _ .. "
each has a family of comets. The idea was
that when tKe 'apTielion distance of a comet
happened to be about the same as the radius
of a planet's orbit, then at some distant
epoch comet and planet passed near each other
and the latter changed the original orbit of
the comet into a smaller one. At the next
passage, the period might be still further
shortened, and so on until eventually the
comet's aphelion point was drawn close to the
planet's orbit. When this had been accom-
plished, the comet was said to belong to the
planet's family.
Russell, 1 however, pointed out that on this
basis of "capture" Jupiter must have done
practically all the capturing. He states that
only one out of the three comets assigned to
Katurn and the two assigned to Uranus comes
1 A. J., 23, 49, 1920.
47
48 COMETS
within 50,000,000 miles of either planet's
orbit. As for the eight assigned to Neptune,
none comes closer than nearly four as-
tronomical units of its orbit! Everyone of
these latter will probably come actually closer
to Jupiter's orbit than to that of Neptune.
The only possible answer to this argument
seems to be that supposing Neptune originally
"captured" these eight comets, for instance,
on later returns, as the comets approached the
Sun, they underwent further perturbation
from the other planets, which so shifted their
orbits that the present state of things came
about. However, the objection does not seem
a strong one, and it would be wiser to give
Jupiter the major credit. Again the chance
that a comet will pass within a given distance
of Jupiter's orbit to that that the comet
would pass to within the same distance of
Neptune's is about 33 : 1 ; the chance that it will
pass within a given distance of the respective
planets themselves is about 460:1. On the
contrary, once a comet is within a certain dis-
tance of Neptune, it will remain longer as both
Neptune and it will move more slowly than
would Jupiter and the same comet were they
passing. This velocity ratio, however, is only
COMET FAMILIES 49
2.4:1, so dividing this into the 460, it still
leaves the chances not far from 200 : 1.
Ruling out, therefore, the families of Saturn,
Uranus, and Neptune as being probably only
apparent, we are left with the large and grow-
ing family of Jupiter alone to consider. We
say growing because new comets are con-
stantly found which fulfill the conditions.
But this is not the whole picture for comctary
"death" or at least disappearance has robbed
Jupiter of a few of its children, notably
Biela's Comet of famous memory.
In addition to having their perihelia near
Jupiter's orbit, a common characteristic of
these comets is low inclination for their orbits
and direct motion. Their periods must be
< 12 years from Kepler's Third Law as the
length of the major axes of their orbits cannot
exceed that of Jupiter's. None of them is a
conspicuous object, though several have been
visible to the naked eye at times. Some of
them have little or no tail, even near peri-
helion. These latter characteristics may be
most readily explained on the fact that their
short periods force them to return every few
years to the vicinity of the Sun. The latter
then has gradually depleted these comets of
50 COMETS
tail forming material until little or none is
left, and also the various forces of disintegra-
tion, explained elsewhere, due to both Sun and
planets work upon them more continuously
and effectively than on comets of long period.
We shall take up briefly the description of
a few of these comets, however, confining our
attention in this chapter to Encke's Comet
only.
JSncke*sCpmet belongs to Jupiter's family
and has the^iistinction of being that with the
shortest period known. It is never a bright
object, so it is due to the above fact, and to
certain peculiarities of its motion, that much
attention has been paid to this inconspicuous
comet.
The first time this object was seen was on
January 17, 1786, when a telescopic comet was
discovered by Mechain at Paris. It was fairly
large, with a bright nucleus, but no tail. It
was noted on only three nights, though seen by
other observers. Naturally an orbit, based on
such a short arc, was most approximate.
The object was next discovered by the most
famous of female astronomers, Miss Caroline
Herschel, on November 7, 1795. It was then
about 5' in diameter and had no nucleus. It
COMET FAMILIES 51
was visible three weeks this time, but its
motion could not be satisfied at all by a
parabolic orbit. The third discovery was ten
years later, when it was found by Thulis on
October 19, 1805. On November 1 a 3 tail
was visible. Again the object was seen for
three weeks. This time an elliptical orbit,
calculated by Encke, gave a period of 12.12
years, making it one of the shortest periods
then known.
Thirteen years later, Pons at Marseilles,
who has so many comet discoveries to his
credit, found a faint telescopic comet on
November 20, 1818. This time the comet
remained under observation for seven weeks.
Encke undertook a rigorous calculation of the
orbit using the new Gaussian method. This
at once gave an elliptical path, the period of
the comet being about three and one-half
years. He was now suspicious that the 1818
comet was a return of the one in 1805, as
the elements were very similar. Further re-
search made him certain that not only had the
1818 comfit bpftn observed in 18Qa-.but~alfle~ki
1795 and 1786. He was not the only one to
perceive these connections, as Arago in France
announced it independently for the 1805
52 COMETS
Comet, and Olbers in Germany extended it
to the two earlier cases.
Nevertheless, to Encke properly goes the
great credit, and his important work on the
comet has been universally recognized by the
naming of the object after him. While other
astronomers made suppositions, he calculated
the perturbations of the object for the three
earlier returns, proving definitely that the same
comet had been seen four times. He next
computed an ephemeris for the return in 1822,
which took place on May 23. He showed
that Jupiter's perturbations would lengthen
the comet's period nine days, and that it would
be visible only in the Southern Hemisphere.
This prediction was brilliantly fulfilled by its
discovery on June 2, 1822, by Riimker at
Parametta, in Australia, who followed it for
three weeks.
One would think that this comet had almost
a habit of staying in view from the Earth
exactly this period of time! Encke was now
able to predict a perihelion passage for Sep-
tember 16, 1825. This time the comet was
discovered as early as July 13, and was fol-
lowed for eight weeks. During this return it
was described as a faint nebulosity about
COMET FAMILIES
53
1J in diameter. Its next perihelion passage
was on January 9, 1829, but it was discovered
on October 23, 1828. Six weeks later it was
easily visible to the eye as an object of 5
magnitude.
TABLE 3
ABRIDGED-FORM OF ENCKE'S CALCULATIONS
RETURN
PERIOD
RETURN
PERIOD
year
days
year
day,
1786
1789\
1792J
1795
1799\
1802J
1805
1809)
1812[
1815]
1819
1822
1212 7
1212 67
1212.55
1212 44
1212 33
1212 22
1212.10
121200
1211.89
1211.78
1211.67
1822
1825
1829
1832
1835
1838
1842
1845
1848
1852
1855
1858
1211.55
1211.44
1211 32
1211.22
1211.11
1210.98
1210 88
1210.77
1210.65
1210.55
1210.44
The comet was observed in 1832, 1835 and
1838. On this last return it was discovered on
August 14, perihelion passage being on Decem-
ber 19. This time it remained under observa-
tion for the long period of sixteen weeks. It
was due to this return that the announcement
was made by Encke that, even after every
54 COMETS
possible planetary perturbation had been
allowed for, the comet returned each time to
perihelion two and one-half hours too soon.
Table 3 is abridged from one calculated by
Encke. 2 Allowances are made for planetary
perturbations.
The only possible explanation seemed to be
that the body traversed a resisting medium
which, according to the well-known paradox,
will cause an increase of the velocity and
hence a shortening of the period. On the
basis of this, the "resisting medium" was
rather widely believed in about the middle of
the nineteenth century. But the curious fact
remained that other comets, with a possible
exception or two, showed no such acceleration.
Further, more recent work by Backlund, who
had the advantage of the numerous observa-
tions of many subsequent returns, proved that
the resistance to the motion had decreased,
apparently almost abruptly in 1858, 1868,
1895 and 1904. He also showed that the
retardation seems to occur in a relatively
narrow sector of the orbit near perihelion. It
mounted to an augmention of 0.1 " to the daily
M on. Notices, R. A. S., 19, 7, 1858.
COMET FAMILIES 55
motion during 1819-1859, but to only 0.01"
during 1904-1908.
Nevertheless from 1819 to 1927 the total
diminution of period amounted to nearly
three days, the mean distance decreasing as a
consequence by nearly 300,000 miles. If such
an effect kept up indefinitely we might expect
the comet to finally spiral into the Sun.
Though today we no longer believe in a gen-
eral "resisting medium/ 7 it seems necessary
to postulate some resistance. Perhaps the
best we can do is to assume that the critical
part of the comet's orbit intersects a region
very rich in meteoric material, which, if dense
enough, would produce such an effect. An
alternate hypothesis is that some physical
alteration has taken place in the comet itself.
The writer would incline to the first explana-
tion as being more probable.
This comet, whose recent history is so well-
known, illustrates splendidly the changes that
take place in the coma of such objects.
A few figures will illustrate this point. On
October 28, 1828, when 135,000,000 miles
from the Sun, its diameter was 312,000 miles;
on December 24, 1828, perihelion passage being
sixteen days later, it was onlv 14,000 miles.
56
COMETS
At perihelion in 1838, when 32,000,000 miles
from the Sun, the diameter was only 3000
miles. The dimensions for this return will be
quoted at length as typical 3 (see table 4).
The last return of Encke's Comet was in
1927, when van Biesbroeck found the comet
TABLE 4
DIMENSIONS OF ENCKE'S COMET IN RETURN OF 1838
DATB
R
DIAMKTKR
185*
October 9
1.42
milet
278,000
October 25
1.19
119,000
November 6
1.00
80,000
November 13
0.88
75,000
November 16 ,..,., r
83
62,000
November 20
0.76
55,000
November 23
0.71
37,000
December 12
0.39
6,500
December 14
0.36
5,500
December 16
0.35
4,200
December 17
0.34
3,000
on November 13, its magnitude being 16.0.
He later found images of it on plates of
October 19 and 20, when its magnitude was
17. On December 21 it had risen to 12
magnitude. On January 21, 1928, the comet
had a sharp nucleus, with a broad tail 5' long.
* Guillemin, World of Comets, 242, 1877.
COMET FAMILIES 57
The total light equaled magnitude 8. By
February 1, it had risen to magnitude 6; the
coma was 2' in diameter and round, there
was a nearly stellar nucleus, not in the exact
center. The perihelion passage took place on
February 19. According to Crommelin, there
was no trace of an acceleration on this last
return. 4 The comet was still observable from
the Southern Hemisphere during March.
The statement is met with that Encke's
Comet is about as bright as when first seen,
but a very complete study by Vsechsviatsky 5
showed that the rate of decomposition of the
comet amounted to one magnitude per cen-
tury. Obviously in a few centuries it must
become wholly invisible, if his conclusions are
true.
The comet has been missed at no return
since 1819, the last being the thirty-seventh
return which was observed.
4 Observatory, 51, 137, 1928.
1 Russian Astr. Jour., 4, 208, 1927.
CHAPTER V
THE TAILS OF COMETS
"All as a blazing starre doth farre outcast
His heavie beames, and flaming lockes dispelled,
At sight whereof the people stand aghast"
SPENSER.
With two known exceptions, a nebula and
an M-type giant star, the tail of a great comet
is the most bulky thing in the universe. De-
spite this immense volume, its mass is very
small; so small indeed that only semi-intelli-
gent guesses can be made as to what it is.
Yet it is one of the most beautiful and interest-
ing phenomena to be found in the whole realm
of astronomy.
All comets do not have tails: in fact a tail
~ -- x
is an appendage wBfcli, wfCK^the brighter
comets, grows and develops only in the
neighborhood of the Sun. It is probable, if
not certain, that in the zone of the outermost
planets no comet has one. Some of the
smaller and less conspicuous comets appear to
have none at any time, or at best very short
and faint tails.
In examining a comet visually, or the
58
THE TAILS OP COMETS 59
photographs of one, it is not possible to draw
an absolutely sharp limit and say that here
the coma ends, there the tail begins. For the
tail extends out from the coma, but where they
join there is an inevitable blending of their
light. The most striking fact is, however,
that the tails of all comets point, in general,
away from the Sun. Hence as the comet
approaches perihelion, the tail streams after,
like the smoke from a moving vessel. But
after perihelion, when the comet is leaving
the Sun, the tail goes first.
The dimensions of tails will be discussed for
specific comets in detail; here it suffices to
say that in certain cases they have been known
to be from 50 to 150 million miles long, when
at their greatest development, and, at the end
away from the Sun, to be from 5 to 10 million
miles across. Certain phenomena lead us to
think that they have more or less the structure
of a flat, curved, hollow cone; i.e., that there
is more material in or very near their apparent
boundaries than there is inside. The word
fan-shaped is often appropriately used to
describe certain types.
It is evident that the material which forms
a comet's tail must come from the coma.
60 COMETS
And further that such material is acted upon
by a repulsive force or forces whose seat is in
the Sun. Consequently any particle in the
tail must be subject to at least two forces; the
gravitation of the Sun which produces an
acceleration toward that body; and the re-
pulsive force which tends to drive the particle
away. Hence its actual motion at any instant
is the vector sum of these two. Observation
proves that the repulsive force is greater.
This means further that the tail material is
lost by a comet, and will never be regained.
So unless a comet is furnished in some obscure
way with a fresh supply it is only a question
of time before its tail material will be used
up. All evidence is that there is no compen-
sating gain, at least of any equivalent amount,
so that such material must be lost on every
perihelion passage.
In some comets, which evidence consider-
able activity, when near the Sun, jets and
streamers of light or a series of envelopes are
developed which are concentric. During
these eruptions the nucleus charges con-
tinually in apparent size and brilliancy,
usually growing smaller and brighter just
before the liberation of each envelope. The
BHOOKS'S COMET
Photographed by Barnard at Yerkes Observatory, October 19, 1911
THE TAILS OP COMETS 61
ejection of a jet, however, seems to set up a
sort of oscillation of the nucleus. These en-
velopes seem to be composed of particles,
forming apparently sheets of light, which are
expelled from the nucleus in arcs ranging from
180 to 270. The center of the arc is gen-
erally in line with the axis of the tail. The
mode of action seems to be that these particles
are expelled from the nucleus, but, at a certain
distance out, those toward the Sun are turned
back by its repulsive force. The result is that
they stream sideways on both sides, eventually
getting back into the tail proper. With
Donati's Comet of 1858 there were intervals of
from four to seven days between the formation
of envelopes. But with Morehouse's Comet of
1908, the envelopes lasted only a few hours
and actually contracted as they grew older;
(see page 166). Several were indeed visible
at once. These envelopes seem not to be
sections of the surface of spheres, but of
paraboloids of revolution.
In Comet 1862 (II) according to drawings
by Secchi, the envelope was complicated by
two actual jets of matter which, expelled
toward the Sun from the nucleus, were turned
back in graceful curves, not unlike the jet
62
COMETS
from a hose, held at a 60 angle with the
ground.
From what has been said it seems that the
Sun sets up activity in the nucleus of the
comet, which leads the latter to expel par-
To SUN
FIG. 2
tides. Those of the latter, moving toward the
Sun, are eventually turned back. Hence all,
no matter in what direction they were origi-
nally expelled, go to form the tail. Figure 2
shows an ideal case. It should be added that
THE TAILS OF COMETS 63
the sunward side of the nucleus is the one on
which this activity is usually found.
We have said that the tail is opposite to the
Sun. This is true, but there is always a lag
with regard to the line Sun-comet. The
angle made by the tail with this line varies
for different comets, for the same comet on
different dates, and sometimes we even find a
comet having two or more distinct tails at
once, making quite a large angle with one
another. The curvature is practically en-
tirely in the plane of the orbit. Therefore to
an observer in this plane even a sharply curved
tail appears straight.
The Russian astronomer, Brdikhinc, set
forth a theory, which though modified, has
been the chief basis of our present opinions.
From the study of forty or more comets,
having one or more tails each, he came to the
conclusion that three types of tails covered
most cases. He assumed a repulsive force,
acting inversely as the square of the distance,
of an unknown character. But that while
able to push back the very small particles
which would form the tail, this force was too
feeble to affect the motion of the head of the
comet. This would lead us to infer a rather
64 COMETS
hollow tail, which inference is often shown
to be correct in that the middle of the tail, as
we see it, is darker than the edges. His
results are shown in table 5.
Br&likhine inferred further that the re-
pulsive force was proportioned to the molec-
ular weights of the tail particles. On this
basis he thought type I to be of hydrogen,
TABLE 5
TABULATED RESULTS OP BR^DIKHINE'S THEORY
TYPB
REPULSIVE FORCE +
SUN'S ATTRACTION
INITIAL TELOCITY
I
II
III
18
0.5 to 2. 2
0.1 to 0.3
3.0 to 10.0 km/sec
0.9 to 2.0 km/sec
0.3 to 0.6 km/sec
type II of carbon, etc., type III probably of
iron. In addition, for a few comets he found
a repulsive force of 36; for these he assumed
an unknown gas lighter than hydrogen, an
assumption now wholly discredited. For
anomalous tails, which sometimes point in
the general direction of, rather than from the
Sun, all we have to do is assume that the
particles composing them are heavy enough
for gravitation to predominate.
THE TAILS OF COMETS 65
Historically, Kepler and later Euler sug-
gested that the pressure of sunlight wais t]xe
cause of comets^ tallsT ~Olbers in 1812, and
then "Besset*, ~"BroiigHt "in electrical forces,
assuming Sun and comet had charges of the
same sign. Later came the "cathode" ray
theory, which was suggested by the spectro-
scope. According to this comets' tails are
only streams of cathode rays sent out by the
Sun. However, the preponderance of opinion
now is that the principal agent is the radiation
pressure^ of sunlight. "" "~~
"The" existence 6F this force, so feeble for
bodies of ordinary size, was proved by P.
Lebedew in 1900, and independently by E. E.
Nichols and G. F. Hull in 1901 by laboratory
experiments. We should add that gravitation
of course is proportional to the mass of the
body in question, which in turn equals volume
times density. Light pressure on the con-
trary, in the nature of the case, can be only
proportioned to the area. For any body
volume depends on the cube of a dimension,
area upon the square. Let us now take a very
small object, in the shape of a regular cube,
with edge equal to a. Then g a 8 ; p a 2 .
Suppose this is of the critical size where g =
66 COMETS
p exactly 1 then for a cube with edge = TV,
we find g 1T ?U; p T T , obviously therefore if
0o = p , p = lOgr, or the light pressure is ten
times as great as gravitation in the second
case. On the contrary for a cube with edge
10a, the opposite would be the case. For a
spherical dust particle ToVo~o inch in di-
ameter, p = 100. We cannot push down the
sizes of particles indefinitely, and, as we ap-
proach the size of a wave-length of light,
Schwarzschild has shown that the pressure is
a maximum for particles whose diameters
equals */3, X being wave-length of incident
radiations. Bosler 2 says: "We might then
believe that radiation pressure is not ex-
ercised upon gases whose molecules are
scarcely 10~ 8 cm. in diameter, and therefore
smaller than the limit above. However, gas
gives place to a selective absorption which
just compensates the inverse influence of
diffraction and Lebedew has succeeded in
showing by a very delicate experiment that
does exercise its pressure upon gaseous
to the" very feebTe'
1 Equality of solar gravitation and light pressure exists
for a sphere of diameter = 0.0015 mm.
1 Astrophysique, 424, 1928.
THE TAILS OF COMETS 67
absorbing power of gas (that is, its trans-
parency) the force thus manifested is mini-
mized, hardly indeed ^ P art of the corre-
sponding force on solid particles. Again if it is
a question of gas at atmospheric pressure and
cometary matter, if the latter is gaseous, it
is surely at very much lower pressure."
The discoveries of Debys and Lebedew leave
unanswered the question as to whether comets'
tails are formed of solid dust particles or
gaseous molecules.
Fabry, discussing the probability of comets'
tails being gaseous, calculated that a tail
which was 160,000 miles thick and had a density
of 10~ 11 (compared with air) would be plainly
visible if it diffused light in the same way that
skylight is diffused. But along with mole-
cules of gas, there seems no doubt that dust
particles must be mingled. Further, streams
of particles, actually expelled from the Sun,
may play a part.
Due to faintness, the spectra of tails are
difficult to obtain. They show bands. These
latter seem to be due to the third negative
group of carbon and the negative group of
nitrogen, at very low pressure.
We have seen that particles, which later
68 COMETS
form the tails of comets, are driven away from
the comets' heads by repulsive forces. Such
a particle will move in a hyperbola but not in
that branch which has the Sun for its in-
terior focus. It must move in the other
branch, convex to the Sun. In some tails
there are knots or condensations which are
sufficiently distinctive to be recognized on
succeeding days. It has been possible there-
fore to measure their positions on different
dates, and hence calculate their velocities.
The result proves that they move faster as
they leave the comets' heads. In recent
years Morehouse's Comet in 1908 and also
Halley's Comet furnished excellent examples.
Such measures are difficult to make very
accurate, due to the irregular and changeable
shape of the knots. In the latter case Curtis
found the velocities given in table 6 for
various knots that he identified on successive
days (distance in astronomical units). On
May 31 the comet was 97 million miles distant
from the Sun, and the tail averaged about
18 million miles long during the above interval.
However, Halley's Comet had a perihelion
distance of 55 x 10 8 miles, while some comets,
as for instance Comet 1843, had a perihelion
THE TAILS OP COMETS
69
distance of only about 500,000 miles, passing
actually within 78,000 miles of the Sun's
surface. Naturally this meant tremendous
activity in tail formation. This comet when
nearest the Sun had a velocity of 330 miles/
sec, and took only 2 h ll m to go through 180
TABLE 6
VELOCITIES FOUND BY CURTIS FOR VARIOUS KNOTS ON
SUCCESSIVE DAYS
1910
DISTANCE
FROM
NUCLEUS
KM/SEC
May 12, 14
003
5
May 27-28
.004
13
May 25-26
.010
19
June 2-3
.015
32
May 31 June 1
019
35
May 26-27
.027
38
June 6-7
.044
70
May 30-31
.071
71
June 7-8
.090
91
of its orbit. Its tail was 150 to 180 million
miles long. 3 This means that, were the tail
made up of relatively stationary particles, one
at the end, (taking the lower estimate) would
move through 47 x 10 7 miles in 141 minutes
1 Flammarion, Astr. Populaire, 626, 1881.
70 COMETS
or 56,000 miles/sec; a speed over one-fourth
that of light. Bosler 4 remarks that this is only
an illusion, as it was not the same matter
which one perceived at the same distance out
from the nucleus at successive instants. The
reason given by him and many others is,
inferentially, that such velocities are out of all
reason. It seems to the writer that this often-
repeated statement met with in texts has no
sound basis for this reason: let, in figure 3,
Fia.3
Ci and C 2 be positions of this comet, C 2 just
after it has moved through an arc of 180 in
141 minutes. Let the tail Ci Pi be supposed
equal in length to C 2 P 2 . Now it is geo-
metrically evident that no particle, already
repulsed at Ci and which has gone an appre-
ciable distance towards PI can possibly, by
any juggling of construction, get into tail
C 2 P 2 . Hence the minimum distance a par-
ticle at P 2 would have to travel, would be
4 Astrophysique, 419, 1928.
THE TAILS OP COMETS 71
along some curved path from Ci to P 2 . At a
minimum this is > dP 2 in length. This
proves that the average velocity of P 2 must
O P
have been STHR or 18,000 miles/sec, which is
only in the ratio of 1 :TT to the velocity which
we would have were we to consider PI actually
revolved around to P 2 . While indeed three
times smaller, it is a quantity of same order.
It happens that this velocity is nearly that of
the alpha rays, which consist of positively
charged particles having a mass about four
times that of the hydrogen atom.
It is obvious that the apparent length of a
comet's tail as seen from the Earth is, in a
given case, no indication of its real length.
Also as stated its curvature, being almost
wholly in the plane of its own orbit, is nearly
always seen by us projected into a different
shape. Tails are usually described as being
transparent, that is stars seen through them
lose no brightness. Yet Sir William Herschel
assures us that, for the comet of 1807, stars
seen through the tail lost some of their
lustre, and one near the head was only faintly
visible by glimpses. 6
'Phil. Trans., xcviii, 153, 1808.
72 COMETS
As typical of the mass of a tail take Halley's
Comet. When the tail was long, Schwarz-
schild calculated that if it were composed of fine
dust particles it might have a weight of 1
million tons, but if only of gas molecules then
probably not over 100 tons. (Both constit-
uents are probably actually present, but in
what proportion is unknown.) He further
calculated from the observations on its bright-
ness that there could not have been more
material in 2000 cubic miles of the tail than in
1 cubic inch of ordinary air, and possibly
even less. The density of no artificial vacuum
can come anywhere near being so low as this!
While it is certain the Earth must fre-
quently, in ancient times, have plunged
through tails of comets, without it being
known or at least recorded, yet twice in the
past century this has occurred; in 1861 and
in 1910. The first case took place on June
30, just a day or so after the comet had be-
come visible to astronomers in the northern
hemisphere. No prediction of the event had
been made but several persons in England in-
cluding Hind 6 recorded a peculiar phosphor-
Chambers, The Story of the Comets, 156, 1910.
THE TAILS OF COMETS 73
escence or illumination of the sky. The
writer knows of one man in America who
observed the same thing, Dr. Milton W.
Humphreys, afterwards professor of Greek at
the University of Virginia, and one of the most
brilliant scholars in the country. Though
barely grown when he saw this phenomenon,
at sunrise from the Virginian mountains, his
keen interest in all science caused him to note
the peculiar glow, which he had never before
seen. Dr. Humphreys personally told the
writer of this, saying that it was many years
before he learned the explanation.
The second case was Halley's Comet on
May 19, 1910. But this time we did not
apparently go through the main tail, but
through a detached streamer.
The writer, then at Lick Observatory,
California, was awake and on the lookout all
the previous night, as were most of the staff.
However, brilliant moonlight effectually ob-
scured unusual phenomena, if indeed there
were any. The writer observed at intervals
with the 12-inch refractor in the region of the
computed radiant of any meteor that might
come from the tail, but saw none. (He would
like to add that in view of our present knowl-
74 COMETS
edge the presence of meteors therein would
be a most surprising phenomenon! He was
not, even then, personally responsible for com-
puting this radiant. The position was found
in some journal.) However, this case was
not comparable to that of 1861 when the
main tail of the comet was doubtless com-
pletely passed through.
Of comets with several distinct tails per-
haps that of 1744 holds the record with six,
while the great Comet of 1825 comes a close
second with five. It is true that Borelly's
Comet of 1903 showed nine on a Greenwich
photograph, but several were very faint. 7
The daylight comet 1910a for several nights
showed two distinct ones, with very different
curvatures, the longest being at least 30 in
length. Biela's Comet had a companion with
a tail or tails of its own. Late in February,
1846, it had three tails, a tripod tail as Maury
expressed it (see p. 137). Many other cases
could be quoted. It is, however, well to call
attention to the fact that on photographs of
bright comets the tail is not uniform all the
way across, but often seems made up of
Mon. Not., R. A. S., 64, 84, 1903.
THE TAILS OF COMETS 75
bright, long, streamers, with darker spaces
between. To the writer the streamers as they
approach the wider end, on some photographs
of Halley's Comet, strongly remind him of
auroral streamers in relative position and
shape. According to Chambers 8 the follow-
ing rather recent comets had a second tail,
sometimes called a "beard" which reached out
more or less toward the Sun: Comet of 1823,
1848 (II,) 1851 (IV), 1877 (II) and 1880
(VII). He further states, on the authority of
Valz that the main tails of comets 1863 (IV)
and (V) deviated from the planes of their
orbits, and that only two other similar cases
were known.
The question of "vibration" or "flickerings"
that have been reported as going from end to
end of certain tails at short intervals is doubt-
less purely a meteorological effect, and de-
serves no discussion here.
Comet Brooks (191 Ic) and Comet Gale
(1912 a) each furnished a chance to compare
Brddikhinc's theory with facts. According to
Baldct's work, using an objective prism
camera, the tail of the former could be traced
1 Page 23.
76 COMETS
5 from the nucleus. At this distance it still
had almost the same intensity as near the
head. Its spectra consisted of the third
negative group of carbon. The negative
group of nitrogen was absent. The more in-
tense images were doubled on the plate of
October 31. Also one could distinguish on
this plate clearly two branches of the tail
forming an angle of 25 with one another.
And tKey gave the same spectra! According
to the theory of Br&likhine the tails would be
sensibly types I and II, attributed respectively
to hydrogen and hydrocarbon.
Again Comet Gale had two principal tails,
the one sensibly rectilinear, long and rela-
tively intense, the other curved, feeble, short,
forming with the first an angle which varied
from 65 to 90. These two tails belong to
types I and III of Br&likhine, who assigned
the first to hydrogen and the third to metallic
vapors. But on the plate of October 17 can
be seen the spectrum of the first tail having at
its base, on the edge of the continuous spec-
trum of the nucleus, the very short image of
the second tail about J long. The angle
between the tails on the spectrogram is about
70. Both tails have the same identical spec-
trum of the third negative group of carbon.
THE TAILS OF COMETS 77
These two cases, cited by Baldet, seem to
contradict decisively Br&likhine's theory of
comets' tails so long held as most probable,
unless indeed this were greatly modified.
Bobrovnikoff states that Comet 1914 (V) had
two tails, both showing the same spectrum.
He further adds that no tail has ever shown
the hydrogen or metallic (except sodium)
spectrum.
In discussing Morehouse's Comet, Barnard
makes the following remarks about the re-
ceding masses photographed in its tail. 9 "It
would seem reasonable that these masses
would have a much slower speed than the
individual particles forming the general stream
of the tail. They probably contain particles
of greater size and mass which would be less
under the influence of the pressure of sunlight,
and would, therefore, have a much less out-
ward velocity than the general particles of the
tail. Whatever the cause, the fact is that on
several occasions these masses have had in-
dependent streams or tails issuing from them,
like those from the comet itself, where the
smaller particles are evidently detached from
Aslroph. Jr., 28, 289, 1908.
78 COMETS
the mass and forced out with a very much
greater velocity. This was also shown in the
case of Daniel's Comet in July 11, 1907,
when a mass left behind was actually moving
sunward under the combined influence of
gravitation and the initial velocity of its par-
ticles when component parts of the head. I
have called attention to this peculiarity in the
case of Borrelly's Comet, where the new tail
of July 24, 1903, was moving out more rapidly
than the rear portion of the old disconnected
tail, which was at that time drifting away
from the comet and Sun.
"An example of secondary tails from re-
ceding masses is found on a photograph of
Swift's comet on April 7, 1892. Morehouse's
Comet also presented a similar appearance
on October 15."
Many further details will be given about
the tails of various comets, discussed in other
chapters, which need not be repeated here.
CHAPTER VI
THE SPECTRA OF COMETS
"A blazing star, which is thought by the ignorant to
portend disaster to rulers "
SUETONIUS.
For a complex body like a comet, which
undergoes extreme changes in its nearness
to the Sun, the spectroscope would be ex-
pected to give important data. However,
due to the faintness of many such objects,
and the area over which their total light is
always distributed, observations arc difficult
or impossible with certain types of spectro-
scopes. At present the short-focus prismatic
camera is in high favor.
The oldest spectroscopic observation,
known to the writer, was by Donati. At
Florence on August 5, 1864, he observed
Tempel's Comet 1864 (II) = 1864a using a
stellar spectroscope. Speaking of the spec-
trum he said ". . . . the dark parts are larger
than the luminous part, and one might say
that these spectra are composed of three
bright rays." 1 These arc really three bands,
i Aatr. Nach., 62, 378, 1864.
79
80 COMETS
in the yellow, green and blue, whose heads
have the wave lengths X5630, X5166 and
\4719 respectively. Huggins and Secchi
in 1866 found Tempers Comet, 1865/, had
a continuous spectrum, as well as the three
bright lines. This was the second to be
observed spectroscopically. Again on June
23, 1868, Huggins identified the three bands in
the spectrum of Winnecke's Comet, 1868 &,
with those of the Swan spectrum. Also when
the spectrum of gases obtained by heating
meteorites is observed, the Swan spectrum
appears. Huggins pursued the correct method
of comparing the comet's spectrum directly
with a comparison spectrum. In this way
he was able to observe the coincidences to an
accuracy of about 6 angstroms, the limiting
power of his spectroscope. He added: "The
apparent identity of the comet's spectrum
with that of carbon, resides not only in the
coincidence of the positions of the bands in
the spectra, but also in the very remarkable
resemblance of the corresponding bands in
what concerns their general characters, as
well as their relative light. This is well
recognized in the middle (green) band where
the gradation of intensity is not uniform/'
THE SPECTRA OF COMETS 81
For the next dozen years or more, measures
by different men on comets' spectra brought
different results as to the wave-lengths of the
radiations. This led many to believe in
more than one kind of characteristic spectra.
In 1879 C. A. Young stated that the differ-
ences between results were no more than the
limits of the probable errors, and that there
was no valid reason for supposing the existence
of more than one kind of comet spectrum,
slightly modified in different comets by differ-
ences in pressure and temperature.
However, from 1864 to 1887, out of 23
comets studied spectroscopically, three had
spectra which could not be identified, even
approximately. These were 1865/, 18676,
and 1868a. All these showed three bands,
displaced from the usual positions found in
the spectra of other comets. The observa-
tions on the first two were uncertain, but on the
last, Brorsen's Comet, they were well studied
by Huggins and Secchi. Some dark lines
were detected in the spectrum of Coggia's
Comet of 1874. In 1881 Tebbutt discovered
a comet, 1881 (III). For the first time this
was successfully photographed by Janssen,
and its spectrum was photographed by Hug-
82 COMETS
gins on June 24 and by Henry Draper shortly
after. Huggins's photograph showed the spec-
trum called "cyanogen" and other radiations
near \4050, which he could not identify.
Also there was a continuous spectrum, on
which were some of the principal Fraunhofer
lines. Thus a certain proof was added to that
given by Arago, in 1819, by polarization, that
solar light is reflected by the rarified materials
of comets. Needless to say the photographic
plate today gives us spectroscopic data on
comets, as on all other heavenly bodies, vastly
greater in both amount and accuracy than
any visual observations can give. Sodium
was also first detected in Comet 18816 (1881
(III)), and then in three other comets in
the next two years, all of which had small
perihelion distances. From displacements of
the D lines, the radial velocity of a comet was
first detected in 1882, by Thollon and Gouy,
and by Lohse and Copeland.
From what has been said we see that the
three usual types of spectra are all present or
may be present in comets. This compli-
cates the explanation. However, the presence
of bright lines, or really bands, proves at once
that they contain glowing gas, as these bands
THE SPECTRA OF COMETS 83
could not be merely reflected sunlight. Nor
indeed are such bands to be found in the solar
spectrum. As for the interpretation of the
continuous spectrum, this might of course be
due to glowing liquids or solids in the comet.
It may also be due to reflected sunlight, the
Fraunhofer lines being absent merely from
relative faintness. We have seen that
Fraunhofer lines were indeed present in cer-
tain cases.
Eminent authorities took opposite sides
during the recent past as to whether a comet
was self-luminous or not. At present it seems
that a comet is not self-luminous in the true.
sense7"ttia1rtt^ reasoiTtt""can shine is
due to the Sun's radiations falling upon it.
What may be called the typical comet spec-
trum then consists of a bright continuous
spectrum, on which are superimposed several
bright bands. If the comet comes extremely
near the Sun at perihelion at that time some
bright metallic lines appear, for instance those
of sodium, magnesium, and iron.
There are strange exceptions, however,
Holmes's Comet in 1892 presented a con-
tinuous spectrum only. Also Brorsen's Comet
in 1868 and Comet 1877 (III) were excep-
84 COMETS
tional in not showing the carbon spectrum of
bright bands.
Tebbutt's Comet, 1881 (III), showed at
first almost a pure continuous spectrum; later
cometary bands showed themselves. With
the nucleus becoming less bright they became
more apparent. On June 29 Fraunhofer lines
showed. But on June 24, Huggins, by photog-
raphy, proved the presence in the ultra-
violet of both hydrocarbon bands and solar
absorption lines. This proved the essential
unity of the whole spectrum. Vogel and
Young both traced the carbon bands far down
into this comet's tail.
Prof. F. Baldet, formerly of the Observa-
tory of Meudon, France, has made most ex-
tensive studies of cometary spectra, and the
following discussion is abstracted from his
splendid work. 2
The nucleus presents an emission spectrum,
made up of a series of radiations, which are
always the same, but not yet identified. The
term nucleus is used here, as usual, to mean
not only the image of stellar aspect seen in the
* Bui. Soc. Astr. de France, 39, 577-8, 1925. Also, Annales
de I'Obs. D'Astr. Physique de Paris, VII, 1926.
THE SPECTRA OP COMETS 85
midst of the coma, but also the very luminous
gas which surrounds this for a few seconds of
arc. The nuclei almost always show, with an
intensity more or less great, a special spectrum
characterized above all by a group of radia-
tions about \4050, the identification of which
is still uncertain. He then concludes:
1. The emission spectra of the nuclei of
different comets, at least in all that concerns
the most characteristic radiations, belong to
only a single type. This statement, based
upon the constant ratios of the lines among
themselves and upon their variations in gen-
eral, assumes, however, nothing as to their
unity of origin.
2. With the rather feeble dispersions em-
ployed, it appeared formed of brilliant lines,
and not of the heads of bands as are the spectra
of comas and tails.
Out of 115 unidentified radiations, met in
course of his study there are only 36 which
have not been observed with certainty in
several comets.
The brilliant line at X4313.8 is very fine,
and shows no trace of shading. It cannot
therefore be identified with the fine head of
the band at \4314.3, characteristic of the
86 COMETS
spectra of the hydrocarbons in combustion,
and which is always absent in nuclei. This
spectrum of brilliant lines is as characteristic
of the light emitted by the nucleus as is that
of the Swan bands and of cyanogen for the
head and as that of the bands of double head
of the third negative group of carbon for the
tail. The head, i.e., coma and nucleus, has
the Swan and cyanogen bands. The tail
has an emission spectrum formed by the
doublets of the third negative group of carbon
and sometimes the negative group of nitrogen.
The single exception to this last is the tail of
Comet 1910a. Even this might be explained
by the lack of sufficient dispersion. Also the
doublets of the third negative group of carbon
are never seen in either nucleus or |coma. All
three parts of the comet usually have a more
or less intense continuous spectrum as a
background.
As to the nature of the gases concerned, the
spectrum of carbon has not yet been sufficiently
studied to give a certain answer. This is
because carbonized gas emits different groups
of bands according to the process of excitation,
and, conversely, different carbonized gases can
present the same groups of bands. Whether
THE SPECTRA OF COMETS 87
different gases therefore exist in different
comets can not be affirmed. What is known
is that one or several carbonized gases and
sometimes free nitrogen are found and also
sodium when the comet is quite near the Sun.
The continuous spectrum has a very vari-
able intensity for different comets. It used
to be thought that its explanations lay in the
reflection of the solar light from small
(hypothetical) solid particles. Recently
Fabry has shown the importance of molec-
ular diffusion.
As to the changes of the spectrum
with changing distances from the Sun, the
following is found. At a distance of 2 or 3
astronomical units the spectrum is almost
entirely continuous; the comet simply diffuses
the solar light. In proportion to its nearer
approach the Swan and cyanogen spectra
develop, as well as the peculiar nuclear spec-
trum. At distance unity, with development
of the tail, the third negative group of carbon
is seen. Nearer still the spectrum of the
nucleus rapidly grows dimmer and that of the
tail more brilliant. Next sodium appears, but
to the detriment of the Swan spectrum, which
grows fainter. The most important element
88 COMETS
that complicates this simple succession is the
amount of activity in the nucleus, which is
very variable from one comet to another.
The general conclusion would therefore be
that all comets have the same constitution,
at least approximately.
S. Orlov states: 3 "That at great distances
from the Sun there is no radiation from gase-
ous material. With decreasing distances the
radiations from C 2 N 2 , CH, CO, and Na
appear, only about at 0.1 astronomical unit
do Fe and Ni appear/ 7
Baldet 4 gives his theory of the origin of the
spectra as follows: "We have seen that when
one varies the pressure of the oxide of carbon
obtained in the tube with an incandescent
cathode, the spectrum of the gas modifies.
The most characteristic facts, at very low
pressure, are, on the one hand the disappear-
ance of the angstrom bands as well as the
considerable enfeebling of the third positive
group and of the new spectra which are hardly
visible with ordinary exposures; on the other
Aslr. Jahresb., 29, 1927.
4 Annales de I'Obs. D'Astr. Physique de Paris, VII, 77, 78,
1926.
THE SPECTRA OF COMETS 89
hand the increase of light of the first and third
negative groups of carbon.
"It seems we can explain as follows: the
tungsten cathode, raised to about 2700 ab-
solute, emits a great number of electrons
which form a veritable gas in the tube and
ionize the molecules. The luminous emission
comes at the time from the shock of these
electrons animated with a velocity superior
to the potential of ionization, against the neu-
tral and ionized molecules and of the shock
of the ionized molecules against them and
against the neutral molecules.
"These different kinds of shocks must give
rise to groups of different bands. At very
low pressure, when the shocks of the electrons
predominate, the first and third negative
group alone are visible. At higher pressure,
when the efficacious molecular shocks become
more numerous, the spectrum enriches itself
with the positive groups already mentioned.
The first and third negative groups provide
then for the direct shock of the electrons
against the molecules of CO.
"The effect of pressure is manifested equally
upon the structure of the bands of the third
negative group. At very low pressure, the
90 COMETS
heads are intense and their shading off rapid
they then resemble bands in the tails of
comets
"These experimental results bring an im-
portant argument in favor of the theory of
corpuscular radiation of the Sun given in 1895
by M. H. Deslandres. 5 According to this
theory, the Sun projects cathode rays, i.e.,
electrons, to great distances, which produce
the coronal jets and illuminate -the gases of
comets. We know how well the corpuscular
theory, applied to the polar auroras by M. M.
Birkeland and Stormer, has given a satis-
factory explanation of their peculiarities.
But as a more direct verification of an emission
of electrons to great distances, we have scarcely
noted up to the present any but the appear-
ance of the negative group of nitrogen in
Morehouse's Comet.
"So, since the third negative group of car-
bon obtained at very low pressures, that is to
say under conditions which approximate those
of the comets where the pressure is much
smaller still, is emitted by the direct shock of
electrons, and as it presents a complete
1 Annales du Bureau des Longitudes, V, c. 70, 1897.
THE SPECTRE OP COMETS 91
identity with the spectra of the tails, I have
concluded from it that the carbonized and
ultra-rarefied gases that they contain are very
probably illuminated by the shock of elec-
trons coming from the Sun. (The first nega-
tive group must be equally emitted by the
tails of comets, but it is unobservable because
of the atmospheric absorption in the ultra-
violet.) This conclusion appears to me be-
sides to impose upon itself that the spectrum
identified is very sensible to the influence of
pressure, to the presence of foreign gases, and
to different conditions of excitation that pro-
foundly alter it, mingling with it other bands
or making them disappear, in other words
introducing dissemblances with that of the
comets. However, the possibility of a lumi-
nescence of the tails by the strongly ionized
radiation coming from the Sun, is not at all
excluded "
Bobrovnikoff , 6 basing his work upon spectra
of 22 comets obtained at the Yerkes Observa-
tory with an objective prism, has come to
further interesting results. He finds that
comets have two distinct types of continuous
Astroph. Jr., 66, 145 and 439, 1927.
92 COMETS
spectrum; the first with maximum near X4700
is called "solar type," the second with maxi-
mum near \4000 is called "violet type."
While variations from the average occur for
single comets, it was found that at distance
from the Sun greater than 0.7 astronomical
unit the violet type predominated, at dis-
tances less than 0.7 the solar type. It was
found that far from the Sun the CN IV band
predominates, but near the Sun the C IV
band.
As to jets, both for Halley's Comet and
Comet 1913 (V), their spectrum was found to
be due mostly to cyanogen. These jets
apparently differ from the regular emissions
forming the head of the comet only in the
velocity of ejection, as was shown by a com-
parison with the spectra of the envelopes.
These latter give principally the cyanogen and
Swan spectrum. The ordinary cyanogen
bands never extend far into the tail.
Bobrovnikoff 7 considers the origin of com-
etary spectra under three possible heads: (a)
thermal action of the Sun; (b) solar corpuscular
emission; (c) direct influence of the Sun's rays
*Astroph. Jr., 66, 439, 1927
THE SPECTRA OP COMETS 93
producing fluorescent phenomena. After re-
jecting the first two and accepting the last
explanation as most probably the true one he
adds: "All difficulties of the corpuscular
theory vanish in the case of fluorescence."
And again; "If the theory of fluorescence be
true, comets in the last analysis depend for
their luminosity on the Sun and are intrinsi-
cally dark bodies."
Zanstra, 8 who has done most important
work on this subject, discards the theory of
electric discharges. He concludes that the
bright line and band spectra of comets are
caused by sunlight, which theory satisfac-
torily accounts for the observed brightness of
the head. He considers the mechanism as
nearly always that of resonance, but one case
of fluorescence is known for 5 iron lines in
the Great Comet of 1882.
8 Observatory, 52, 9, 1929.
CHAPTER VII
HALLEY'S COMET
"And. . . . saw the angel of the Lord stand between the
earth and the heaven, having a drawn sword in his
hand stretched out over Jerusalem."
Of all comets there is no possible doubt but
that Halley's Comet has had the most im-
portant influence on astronomy. This comes
not only from the fact that its periodicity was
established before that of any other, as briefly
mentioned in Chapter I, but also because its
history can be traced accurately for over two
thousand years. Its well-timed appearances
at or near several important events in history
has also given it an added interest not only to
the superstitious, but also to the student of
human conduct and reactions.
Edmund Halley, who most justly is immor-
talized by having his name appended to this
famous comet, was born in England in 1656.
He showed great ability in mathematical
studies at Queens College, Oxford, and when
but twenty published a paper on planets'
orbits which placed him high among theoreti-
cal astronomers.
94
HALLEY'S COMET 95
A few years after the coming of the Great
Comet of 1680, Newton published his Princi-
pia, incidentally urged on thereto by Halley,
who also furnished the necessary funds. In
this work Newton explained the theory of
gravitation and applied it to the orbit of the
above-mentioned comet. He set up a method
of geometrical construction by which the
visible part of a comet's path could be repre-
sented, and a parabolic orbit calculated. He
considered it probable that some comets
moved really in elongated ellipses, which
could not be differentiated from parabolas in
the small arc visible to observation. Halley
concurred in the latter opinion.
Halley seriousljyandertook to calculate the
orbits of twentjMour comets which had
enough observations for his purposes. The
catalogue of thoir elements was published in
1705. Table 7 shows his elements, of three of
which we shall speak.
Halley wrote as follows: "Now, many
things lead me to believe that the comet of the
year 1531, observed by Apian, is the same as
that which, in the year 1607, was described
by Kepler and Longomontanus, and which I
saw and observed myself, at its return, in
96
COMETS
1682. All the elements agree, except that
there is an inequality in the times of revolu-
tion; but this is not so great that it cannot be
attributed to physical causes. For example,
the motion of Saturn is so disturbed by the
other planets, and especially by Jupiter, that
his periodic time is uncertain, to the extent of
several days. How much more liable to such
TABLE 7
ELEMENTS OF H ALLEY'S COMET
COMET 1531
COMET 1607
COMET 1682
12
49 25'
50 21'
56 16'
i
17 56'
17 2'
17 56'
f
301 39'
302 16'
302 53'
0.56700
0.58680
0.58328
Retrograde
R
R
perturbation is a comet which recedes to a
distance nearly four times greater than Sat-
urn, and a slight increase in whose velocity
would change its orbit from an ellipse to a
parabola? The identity of these comets is
contirmedi by the fact that in the summer of
the year 1456 a comet was seen, which passed
in a retrograde direction between the earth
and the sun, in nearly the same manner; and
although it was not observed astronomically,
HALLEY'S COMET 97
yet, from its period and path, I infer that it
was the same comet as that of the years 1531,
1607 and 1682. I may, therefore, with con-
fidence, predict its return in the year 1758.
If this prediction is fulfilled, there is no reason
to doubt that the other comets will return."
As Halley died in 1742, at the age of eighty-
five, he of course did not live to see how his
prediction came out, nor could he have ex-
pected to.
As the year 1758 drew near, the attention of
astronomers was turned to Halley's predic-
tion. It is a curious fact that Frenchmen
rather than Englishmen undertook seriously
to compute what had happened to the comet
011 its long journey out and back, and when
its time of perihelion would be. While Halley
had been able to see and indeed estimate
roughly the effects of Jupiter and Saturn
(Uranus and Neptune being then unknown)
yet the theory of perturbations was not ad-
vanced enough for him to come to definitive
results. Incidentally the intervals between
the 1531, 1607, and 1682 returns had been
27,811 and 27,352 days, showing that succes-
sive periods differed by 459 days.
So Clairaut, a French astronomer, under-
98 COMETS
took the laborious calculations laborious
indeed for there were no computing machines
in those days, and few convenient tables for
calculation! He was assisted by Lalande and
Madame Hortense Lepaute, and they just
managed to get the first report ready for the
Academy of Science in November, 1758.
Clairaut says: "The comet which has been
expected for more than a year has become the
subject of much curiosity True lovers of
science desire its return because it would
afford striking confirmation of a system in
favor of which nearly all phenomena furnish
conclusive evidence. Those, on the contrary,
who would like to see the philosophers em-
barrassed and at fault hope that it will not
return and that the discoveries of Newton and
his partisans may prove to be on a level with
the hypotheses which are purely the result of
imagination. Several people of this class are
already triumphing, and consider the delay of
a year, which is due entirely to announce-
ments destitute of all foundation, sufficient
reason for condemning the Newtonians. I
here undertake to show that this delay, far
from invalidating the system of universal
gravitation, is a necessary consequence aris-
H ALLEY'S COMET, MAY, 1910
Photographed by II. D. Curtis at Lick Observatory
HALLEY'S COMET, MAY, 1910
Photographed by H. D. Curtis at Lick Observatory
HALLEY'S COMET 99
ing from it; that it will continue yet longer,
and I endeavor to assign its limit."
Clairaut indeed assigned a delay of 618 days,
518 due to Jupiter, 100 due to Saturn. This
placed the date of perihelion passage in
April. Due, however, to neglected terms in
the equations and unknown bodies, he said
the date might be out by one month. It was
discovered on Christmas night, 1758, by an
amateur astronomer, Palitzsch, near Dresden.
It was seen in France on January 21 by Mes-
sier who observed it for three weeks. Messier
was Delisle's assistant, the latter being direc-
tor of the Observatory of Paris. Delisle from
some contemptible motive 1 refused to allow
Messier to announce his observations, and
the comet was lost in the twilight as it neared
perihelion. Delisle and the innocent Mes-
sier were both punished as it was with great
difficulty that the French Academy were
convinced that the observations were not
forgeries, when once the stupid director
brought them forth.
The comet passed perihelion on March 12,
1759, or within thirty-two days of the pre-
1 Chambers, The Story of the Comets, 117, 1910.
100 COMETS
dieted date. The delay had been only 586
instead of 618 days. The work of the three
French astronomers, which meant six months
of the most intense labor to calculate the per-
turbations by the methods then in use, justly
ranks them next to Halley himself. After
perihelion passage, the comet was seen
throughout April and May but best in the
Southern Hemisphere.
The importance of this return of Halley's
Comet cannot well be overestimated. It
proved that some comets at least are definite
members of the Solar System, and that New-
ton's laws fitted their motions as well as those
of the planets. Also Halley's prediction was
triumphantly vindicated.
The next return was due in 1835. Mean-
time theoretical astronomy had made great
advances, and Uranus had been discovered,
adding another body whose perturbing force
had to be reckoned with. In 1820, Baron
Damoiseau of Paris gained the prize offered
by the Academy of Sciences of Turin for the
best paper on the perturbations of the comet
since 1759. He set November 4, 1835, as the
date of perihelion passage. Comte de Pont6-
coulant, following similar lines, predicted
HALLBY'S COMET 101
November 12, 1835. Both neglected the in-
fluences of some of the smaller planets. Later
a German, Rosenberger of Halle, with the
thoroughness characteristic of his country-
men, decided to investigate the comet's orbit
all the way back to 1682, allowing not only
for the major planets, but also Mars, Earth,
and Venus. He added some allowance for the
hypothetical "resisting medium" then thought
to affect Encke's Comet, by Encke himself.
He came to two results: without "resisting
medium" effects, November 11, 1835; with
such effects November 3, 1835. Another
German, Lehmann, feeling that there was
still room for improvement, worked back to
the 1607 return. He decided on November
26 as the date of perihelion passage. The
average of November 4, 12, 11 and 26 is
November 13; the actual date was November
16, 1835, which meant 28,006 days had elapsed
since the 1759 perihelion.
Calculations for the next return, which
were promptly taken up, showed that Jupiter
would shorten the period of the comet about
800 days. Pontcoulant indeed calculated
the perturbations and the perihelion passage
for 1910, and published his results as early as
102 COMETS
1865. 2 In this the date of perihelion was
determined as May 24.37. However, as the
time for the return drew near, Cowell and
Crommelin in England, examining into Pont6-
coulant's published results, found some im-
possible figures. This and the importance of
the problem induced them to undertake a
complete recomputation. This was done in
such a thorough manner that it will long re-
main a model for similar cases. They de-
duced April 8 as the date of perihelion, a
month from that deduced by Pont6coulant.
The actual date was April 19, 1910.
However, they pushed their researches in
another direction, which was to determine the
perturbations for past centuries, and thus
determine which of the comets of antiquity
were certainly returns of Halley's Comet.
Hind 8 had long before made many identifica-
tions; also Langier, 4 Pingr6 and Burckhardt
should be mentioned in this same connection.
The dates of return, so far identified, were in
B.C. 240, 87, 11; A.D. 66, 141, 218, 295, 373,
451, 530, 607-8, 684, 760, 837, 912, 989, 1066,
* Comptes Rendus, 58, 826 and 015.
The Story of the Comets, p. 50 et seq.
Comptes Rendus, 23, 183, 1846.
HALLEY'S COMET 103
1145, 1223, 1301, 1378, 1456, 1531, 1607, 1682,
1759, 1835 and 1910. Crommelin states that
a comet seen in 467 B.C. was not improbably
Halley's Comet, but the data are insufficient
to be certain about it. It will be noticed
that the only missing date after 240 B.C. is
about 163 B.C. For the earlier returns we are
indebted mostly to the Chinese for the obser-
vations, for the later ones to Europeans. As
we have briefly sketched the history of the
comet from the standpoint of its orbit, we will
now take up these various returns from the
historical and descriptive side.
Chinese records give a comet appearing in
240 B.C., which according to the work of
Cowell and Crommelin was Halley's Comet.
No identification is possible for 163 B.C.,
though comets were seen in 166 and 165 B.C.
In the Theatrum Cometicum of Lubienietz
there is mention of a bright comet in 87 B.C.
The Chinese recorded a comet visible in
August, and modern calculations put the
perihelion passage of Halley's Comet during
that month. There seems no doubt of the
identification. In 11 B.C., the Chinese made
observations for nine weeks on a comet, from
which a rough orbit was calculated by Hind.
104 COMETS
The elements of this orbit are sufficiently near
to those of Halley's Comet to leave practi-
cally no doubt of its identity. Recent work
has confirmed Hind's conclusions. The comet
of 66 A.D., mentioned elsewhere (p. 11) was
seen by the Chinese for seven weeks. Hind
again calculated a rough orbit and concluded
this was Halley's Comet. It was this appear-
ance that started the career of the comet as a
prophet of terrible calamities, for Josephus
considered it a forewarning of the destruction
of Jerusalem and so large a part of the Jewish
nation.
The 141 A.D. comet, which was seen for four
weeks in China, and also that in 218 which
was visible for six weeks, were certainly
Halley's Comet. The identification with the
comet of 295, visible seven weeks, and seems
less certain though very probable. More
recently Hirayama, 5 using Chinese observa-
tions, has proved that a comet which passed
perihelion on Feb. 13, 374, was certainly Hal-
ley's Comet. The comet of 451, visible for
thirteen weeks in China, was evidently a
striking object. Its orbit, calculated by
Observatory, 34,
HALLE Y'S COMET 105
Langier, proved it to have been Halley's
Comet. It appeared indeed at one of the
terrible crises of the world's history, for this
was the date of the Battle of Chalons. In this
battle the Huns under their king, Attila, were
finally defeated by the allied Christian armies
under the Roman general Aetius. Bad as
were the Middle Ages, had the Huns con-
quered, civilization would indeed have been
put back for centuries at least.
A great comet appeared in 530 or 531, of
which it is recorded that it was very large and
fearful, being seen in the west for three weeks.
Its rays extended to the zenith. Hind identi-
fies this as Halley's Comet. A comet ob-
served in China in 607 is thus identified by
Cowell and Crommelin. But the Chinese
records are here most confusing. Hind
thought the comet of 608 to be the proper one;
in any case we may be sure one or the other
of the comets of the years 607-8 was Halley's
Comet. Comets in 684, 760, and 837 proved
to be returns of Halley's Comet. The comet
was visible five and eight weeks respectively
for the first two. The Chinese record four
comets for 837, so that year must have been a
remarkable one for such bodies. The first
106 COMETS
of these was Halley's. The identity of Hal-
ley's Comet in 912 caused trouble. Hind's
result for perihelion passage differs four
months from that of Cowell and Crommelin.
The latter stated that the computed position
could not be certainly identified with that of
any comet recorded in that year. But a
recently discovered Japanese manuscript gives
observations which make the identification
satisfactory.
In 989 the Chinese recorded observations of
a comet visible for five weeks. Burckhardt
calculated its elements, which proved its
identity. This comet is mentioned also by
Anglo-Saxon writers. The return of the
comet in 1066 was noteworthy because in that
year the Normans under William the Con-
queror invaded England. Also because we
have the oldest contemporary picture of the
comet which is embroidered in the famous
Bayeux Tapestry, said to have been the work
of Queen Matilda, William's wife. Zonares,
a Greek historian, said it was "large as the full
Moon, at first without a tail on the appear-
ance of which it diminished in size." The
Chinese say that it was visible for sixty-seven
days after which "the star, the vapor and the
HALLEY'S COMET 107
comet all disappeared." There seems no
doubt that the comet was a brilliant and
striking object, judging by the large amount
of attention it received.
The 1145 return, which took place in
April and May, was not so striking. In
August and September, 1222, it was recorded
in England as a star of the first magnitude,
with a tail. In 1301 the comet was a brilliant
object, visible for six weeks, and observed in
both China and Europe. In 1378, though
extensively observed, it seems to have been
less striking. In 1456 the Chinese described
it as having a 60 tail, with a head that was
at one time round and the size of a bull's eye,
the tail being like a peacock's. It was visible
for a month, perihelion passage taking place
on June 8. Halley thought this was a return
of the comet, when he was investigating the
subject, and Hind later proved it.
As Constantinople had been captured by
the Turks in 1453, they then appeared in grave
danger of overrunning and conquering all
Europe, so that the appearance of Halley's
Comet caused the greatest terror. The state-
ment that the pope, Calixtus III, issued a bull
excommunicating the comet has been proved
108 COMETS
false, though it has found its way into many
books. The story appears to have had its
origin in a paragraph of Platina, in his Vitae
Pontificum, 1479. He was then in Rome and
archivist of the Vatican. He wrote: 8 "A
hairy and fiery comet having then made its
appearance for several days, as the mathemati-
cians declared that there would follow a
grievous pestilence, dearth and some great
calamity, Calixtus to avert the wrath of
God ordered supplications, that if evils were
impending for the human race, he would
turn all upon the Turks, the enemies of the
Christian name. He likewise ordered, to move
God by continual entreaty, that notice should
be given by the bells to all the faithful, at
midday, to aid by their prayers those engaged
in battle with the Turk."
Nevertheless, though no bull was issued,
the pope evidently thought it prudent to in-
voke the Deity against the probable ills to
follow the comet's appearance. Nor is he to
be blamed for having the same scientific
beliefs as other educated men of his times.
The comet on its return in 1531 was visible
Pop. Astr., 16, 482, 1908.
HALLEY'S COMET 109
for five weeks, and in 1607 for nine weeks.
Its next return in 1682 was observed by Hal-
ley himself. It was during this apparition
that bright jets, associated with the nucleus,
were seen by Hevelius.
On the return of Halley's Comet in 1835, it
was not discovered until August 6, though
search had been made for many months pre-
viously. Dumouchel, in Rome, was the dis-
coverer, and the comet was found near its
computed position. It was so faint that it
was not seen elsewhere until August 21, when
Struve saw it at Dorpat. His observations
showed that Rosenberger's ephemeris was
wrong by only 7' in right ascension, and 17' in
declination. The comet meantime was rapidly
becoming brighter so that on September 23
Struve saw it with the naked eye. Bessel on
October 22 drew the head of the comet with
wings attached to the nucleus, quite like those
photographed on May 27, 1910. For the
next month a tail, estimated by various Euro-
pian observers at from 20 to 30 long, was
seen. Perihelion passage occurred on No-
vember 15.
Maclear, at the Cape of Good Hope, did
not record any observations until January 24,
110 COMETS
1836. Whether this was because he did not
see the comet earlier which scarcely seems
possible or whether he was occupied with
other duties, is not now apparent. On Janu-
ary 24, he described it as being of the 2.3 or 3
magnitude, with no tail. For the next week
he described some surprising phenomena, as
indeed had been already done by observers in
Europe during the previous fall. We quote
his final statement : "Throughout the succeed-
ing three months the coma went on increasing,
until the outline finally became so faint as to
be lost in the surrounding darkness, leaving a
blind nebulous blotch with a bright center
enveloping the nucleus of variable brightness,
depending on moonlight, or the state of the
atmosphere, and variable distance." The
comet was followed until May, 1836.
On its last return Halley ; s Comet was dis-
covered by Wolf at Heidelberg, on September
11, 1909, when over 300,000,000 miles from
the Sun and even farther from the Earth. It
was photographed the next night at Lick
Observatory, California, by H. D. Curtis, for
whom the writer was then acting as assistant.
Later a search at other places showed that it
was on a Helwan, Egypt, plate of August 24.
HALLE Y'S COMET 111
It was followed, photographically, until June
1911, when it was 520,000,000 miles from the
Sun.
This return being so recent, and the comet
being considered such an important object, the
number of photographs taken at many leading
observatories was immense. Also very nu-
merous visual observations were made, and
many spectrograms obtained. In fact, the
data are so numerous that it is impossible to
discuss them in detail. Only certain points of
interest can be touched upon here. The
writer will add some personal impressions, for
due to the wonderful climate of Mount Hamil-
ton, it is doubtful if anyone besides Dr.
Curtis actually saw Halley's Comet oftener
than himself, up to the time it ceased to be a
naked-eye object in the summer of 1910. As
the writer is familiar with the hundreds of pho-
tographs taken at the Lick Observatory, much
of what is to be said will be based upon these.
After September 12, when the comet ap-
peared as a faint nebulous speck on the plate,
the image indicating a diameter of 13,000
miles only, it rapidly increased in size. Table
8, due to Curtis, 7 gives in condensed form the
Pub. A. S. P., 22, 117-130, 1910.
112
COMETS
TABLE 8
DIMENSIONS OF H ALLEY'S COMET
DATE
COMA
NUCLEUS
TAIL
TAIL*
1909
September 12
13,000"
6,000*
September 13
5,000
September 22.....
November 14
December 13 . .
2,900
2,400
5,600
December 14
220,000
December 16
430,000**
mo
January 7
79,000
2,600
January 28
181,000
4,700,000
February 4
141,000
5,500
8,800,000
February 11
170,000
February 28
153,000
March 3
4,000
April 12
6,400
April 19
129,000
April 21
9,200,000
April 23
2,300
April 28
118,000
April 30
19,000,000
May 1
122,000
1,400
May 5
161,000
800
27,800,000
31
May 9
1,000
30
May 10
24,400,000
May 17
18,900,000
May 19
18,600,000 vis
May 20
400
May 22
300
May 23
194,000
290
16,000,000
May 28
17,600,000
32
HALLEY'S COMET
113
TABLE 8 Concluded
DATE
COMA
NUCLEUS
TAIL
TAIL*
1910
May 31
315,000
June 1
1,000
15
June 2
308,000
21,000,000
June 8
340,000
June 11
2,300
July 1
18,000,000
1911
April
30,000
* As photographed by Ellerman in Hawaii.
diameter of the head, i.e., coma, of the nu-
cleus, and the length of the tail. The con-
tractions of the coma, near perihelion, which is
a usual and well-known pheomenon, is ex-
cellently shown.
It must be understood that both atmos-
pheric conditions and lengths of exposure, as
well as type of telescope employed, keep the
figures for coma and nucleus from being
strictly comparable. For instance, the nu-
cleus would in general be larger when the
comet was faint, from relative over-exposure.
During December, Aitken made a visual
measure of the halo about the head showing it
to be 15' or 550,000 miles in diameter.
114 COMETS
Due to relative position and lag in activity,
the comet was rather a poor object until after
its perihelion passage which took place on
April 19. From then on until summer it was
brilliant, and showed many interesting
changes and phenomena. The inner coma
varied greatly from night to night. Small,
sharp jets, sometimes showing a spiral effect,
were frequent. Bright, strong appendages
and wings, generally asymmetrical, frequently
appeared at times of greatest brightness.
For instance, on May 14, both head and en-
velopes were markedly asymmetrical, being
stronger on the south side. From the elon-
gated nucleus proceeded two remarkable wings,
nearly as bright as the nucleus itself. One
was in the tail's axis and 50" long, the other
at an angle of 59 was 70" long.
An almost explosive change is thus de-
scribed by Curtis: 8 "Both plates were taken
on May 23, one hour and fifty-nine minutes
apart. On the former plates the nucleus is
sharp at the rear, and strong jets proceed from
it on the side toward the Sun at an angle of 60
with the axis of the tail. The jets give a
Pub. A. S. P., 22, 121, 1910.
HALLE Y ; S COMET 115
spiral effect to the head and are so bright at
the nucleus that this appears wedge-shaped.
In the next very bright growths, roughly
spherical, have formed on each side of the
nucleus, with a slightly fainter patch at the
rear which joins the two globes of matter.
From edge to edge the new growth measures
37" = 3570 miles. The photographic diame-
ter of the brightest part of the nucleus on the
first plate is 3" = 290 miles; this may well be
considerably larger than the true nucleus. A
plate exposed from 8:31 to 8:49 P.S.T.
shows the nucleus as still sharp and with no
trace of the appendages. On the next plate
exposed from 8 :55 to 9 :55 P.S.T., the head is
so bright as to mask considerably any details
about the nucleus, which appears less sharp
and roughly spherical, with an estimated
diameter of 12" = 1160 miles. At 10:15
the nucleus is sharp; small faint traces of
the outer parts of the new growth can be
seen, apparently detached from the nucleus.
These globular appendages seem then to
have reached their full growth in the interval
of about 50 minutes, though the interval
may be considerably shorter. Under this
assumption the lower limit of the velocity
116 COMETS
of movement outward within 20" of the
nucleus was 0.88 km. per second, and it may
have been considerably higher. It would
have been of interest to trace this growth
further. May 24 was cloudy."
Bobrovnikoff, 9 using plates available later
from other stations, has followed the question
further as follows:
May 23, 20 h :00, m Many jets around nucleus.
May 24, 3:13, Nucleus rather large. Spectrum un-
usual, steady bright jet.
3 :42, Nucleus still larger. Violet part of
spectrum weak.
4 :10, Nucleus very much larger. Ap-
pendages. Extinction of violet
part of spectrum. Faint halo.
6 :18, Two globular growths on each side
of nucleus.
May 25 , Double nucleus and two systems of
envelopes. Halo brighter.
May 26 , Nucleus double. Halo much
brighter.
In his opinion these striking transformations
may be described as an explosion of the nu-
cleus. The spectrum of the jets was found to
be gaseous with a preponderance of cyanogen.
Astroph. Jr., 66, 145-169, 1927.
HALLE Y'S COMET 117
Halley's Comet was due to pass across the
Sun's disc on May 18, but during the night for
Europe and America. An expedition there-
fore was sent to Hawaii for the special pur-
pose of observing this. Ellerman, observing
with a 6-inch telescope, was able during the
critical interval to see small sun-spots most
clearly but absolutely no trace of the comet's
nucleus could be detected. Had the latter
been a single, solid mass as much as 200 miles
in diameter, it should have been seen without
difficulty.
The Earth was due to pass through the
comet's tail on May 19, so a careful lookout
was kept for any possible meteorological
effects which could have been caused thereby.
W. J. Humphreys 10 of the United States
Weather Bureau sums his investigations,
based on reports from the whole world, as
follows: "The halos, coronas and other phe-
nomena listed above were both widely scat-
tered and, in some respects, distinctly unusual;
and their occurrence coincident, as near as
can be determined, with the passage of the
earth through the tail of the comet suggests
" Pub. Am. Astron. Soc., 2, 17, 1915.
118 COMETS
for them a cosmical origin. Still they were
far from universal, and besides they have all
been seen before when there was no comet to
which they could be attributed; and, there-
fore, while admitting the possibility, in this
case, of a cometary influence, it would seem
rash, without additional evidence, to conclude
that the comet was the principal or even par-
tial cause of any of the appearances men-
tioned."
It may be added that, while no trace of the
effects of entering the tail could be detected
on any of the critical nights, still from its
relative position on former and later dates it
was concluded that the Earth did not pass
centrally through the tail, but only through
its outer edges. Brilliant moonlight, how-
ever, was enough to wholly mask any faint
glows or other similar appearances in the night
skies at the date.
The tail was a very fine object in the morn-
ing sky up to the date of our passage through
it. At Mount Hamilton the following esti-
mates were made by Curtis, 11 the times being
(old) G.M.T.: May 19.0, two branches,
" Pub. A. S. P., 22, 128, 1910.
HALLEY'S COMET
119
northern one stronger. Total width 17.
May 20.0, tail much fainter than the previous
morning, 12 broad in Pegasus. May 20.7,
perhaps 20 of tail visible in west in strong
twilight and moonlight. May 21.0, no trace
of tail in east. May 21.7, 20 to 30 of tail in
west. He last saw the comet with the naked
TABLE 9
BARNARD'S DERIVATIONS FROM RESULTS OF OBSERVATIONS OP
H ALLEY'S COMET
STATIONS
INTERVAL
HOURLY
RECEf
PER 81
JSION
BCOND
From
Comet
From
Sun
Yerkes-Honolulu
h
4.25
i
3.60
km.
37.2
km.
63.9
Yerkes-Beirut
15.15
5.17
53.3
80.0
Honolulu-Beirut
10.90
5.78
59.7
86.4
eye on June 28, when its effective brightness
was 6 magnitude. On May 18, Barnard at
Yerkes observed the tail as 107 long; the
writer and others at Mount Hamilton were
able to trace it to a length of about 130 to
140. In early May the comet's head was
magnitude 2.
On June 6, 1910, a broken part of the tail
could be traced as it receded from the head.
120 COMETS
Barnard 12 on the basis of plates taken at
Yerkes, Honolulu, and Beirut derived the
results shown in table 9.
On the comet's return trip from the Sun,
Innes was able to measure the comet to
August 12, and Perrine to August 26. On this
latter date its total light equalled a star of
magnitude 9. By December it had decreased
to magnitude 12. 13 On January 8, 1911,
Barnard found that it had a diameter of 33",
that it was round, slightly condensed, and
had no nucleus. The magnitude was 13 to 14.
On February 25, at Algiers, it was found to
be of magnitude 14. Barnard in April found
its diameter to be 10" and its magnitude 15.
On May 27, Curtis at Lick photographed it
as of magnitude 16, when it was barely visible
on the plate.
A series of measures of its brightness, 14 dur-
ing its whole apparition, was made at Hel-
wan, Egypt. There the visual magnitude
was consistently found to be one magnitude
brighter than the photographic. It was last
seen on April 29, 1911, but last photographed
" Pub. Am. Astr. Soc., 2, 38, 1915.
" Observatory, 34, 56, 1911.
"/r. B. A. A., 22, 204, 1912.
HALLEY'S COMET
121
on May 18. Its photographic magnitude
238 days before perihelion passage was the
same as that 394 days after showing the lag
in excitation, which is discussed in general
elsewhere (p. 28). Table 10 gives the Hel-
wan photographic magnitudes.
TABLE 10
HELWAN, EGYPT, PHOTOGRAPHIC MAGNITUDES
DATE
MAGNI-
TUDE
DATE
MAGNI-
TUDE
1909
August
15}
1911
January
151
September
15
February 5
15}
October
14J
March (first half)
14}
mo
November
141
March (second half) . .
April
15}
15}
December
15
May
151
Of the many important studies made on
Halley's Comet, a few will now be touched
upon. Orloff 15 worked out the mass of the
nucleus of the comet, basing his results on the
period May 24 to July 4, 1910, during which
he could detect phase effects. These latter
proved that the nucleus shone almost wholly
by reflected light; the intrinsic light being
Bui. St. Petersburg Acad., No. 5, 1913; and Pub. A. S. P.,
25, 175, 1913.
122 COMETS
negligible. His conclusions were based on
two suppositions: (1) that the nucleus was
one solid body, (2) that it consisted of solid
particles 2 mm. in diameter. No smaller parti-
cles could be assumed, for then radiation
pressure would have produced observable
perturbations. Taking the mass of the Earth
as 1, he then found that r > mass of
o x 10 5
nucleus > ^. From this he concluded
that the minimum mass of the nucleus was
30 X 10 6 tons. He proved that in such
studies the phase effect could not be neglected.
As for the mass of the tail, some results 16
by Schwarzschild and Kron are given.
"The reasonable assumption is made that the
particles in the comet's tail are fluorescent
molecules; from many accordant physical
investigations it is shown that the size of such
molecules for the average hydrocarbon gases
is of the order of mm. From this it would
follow that the mass flowing along the cross
section of the tail per second would be only
"Astroph. Jr., 34, 342, 1911; Pub. A. S. P., 24, 138, 1912.
HALLEY'S COMET 123
about 150 grams (5 oz.), while the density, in
terms of the density of air, is infinitesimally
small, only - If the Earth were
TC XN J-vl
exposed for an entire day to a tail of such
density with a velocity of 60 miles sec, the
total mass caught by the Earth in its passage
would be a matter of only a few hundred
tons.
Such a result bears out most forcibly what
has been already said about the extremely
low density that must be assigned to the tail
of a comet.
Baldet found that the head gave radiations
characteristic of comets the spectrum of
cyanogen and the Swan spectrum, with an
intensity sufficient so that one could affirm
that they did not extend into the tail. This
point was confirmed by plates of other ob-
servers. A plate on May 5 showed traces of
the red spectrum of cyanogen. The comet
showed a continuous spectrum sufficiently
marked.
To other observers the spectrum of the tail
showed oxide of carbon at very low pressure.
It resembled in all points that of Comet More-
house. Sodium rays were registered at the
124 COMETS
Lowell Observatory 17 when the comet was
0.69 unit from the Sun. Also the relatively
intense continuous spectrum showed a great
number of Fraunhofer lines.
However, this comet offered a chance to
study variations in spectra, due to its bright-
ness and long period of visibility. Thus W.
H. Wright, on October 22, 1909, obtained a
purely continuous spectrum from X3750 to
X5000. This was six months before perihelion
passage. Deslandres and A. Bernard saw the
first bands of cyanogen appear. Last of all,
near the Sun, the sodium D lines appeared.
Halley's Comet 18 according to a study by
Bobrovnikoff showed distinctly two types of
continuous spectrum, which may be denomi-
nated the solar, with maximum about \4700,
and the violet, with maximum about X4000.
The change took place about 1.2 astronomical
units from the Sun, the violet being character-
istic beyond this limit, the solar within it.
His work was based upon three monochro-
matic images at \3883 (CN IV), X4020^tl00
(C H) and X4737 (C IV). All images reached
" Lowell Obs. Bui., 2 and 3, 1911.
" Astroph. Jr., 66, 145, 1927.
HALLEY'S COMET 125
their minimum size about a month after peri-
helion passage.
He adds: "The reflected solar spectrum
makes its first unmistakable appearance at
about the middle of February. At the end of
this month the solar spectrum is already
stronger than the violet and in April and May
.it is the predominant feature of the whole
spectrum. On May 29 the first signs of the
violet spectrum reappear and on June 11 the
maximum of intensity of the continuous spec-
trum is in the violet, resembling thus the
conditions on February 3. This is in contra-
diction of the general but unwarranted opinion
that when comets are far from the sun they
shine chiefly by the reflected sunlight."
Halley's Comet did not come nearer than
about 5 million miles of the Earth's orbit, but
despite this fact there is a most interesting
meteor stream connected therewith. These
meteors are known as the Eta Aquarids, and
are seen during about two weeks including the
last day or two of April and early May.
These meteors were seen as early as 1870 19
and recognized as a real stream, and various
guesses and partial proofs were made that
" Meteors, Chapter VIII.
126 COMETS
they were connected with Halley's Comet.
It was not until 1909 that the writer made
the necessary observations and calculations
to make the proof complete. He also first
proved the daily motion of the radiant.
Among other interesting facts brought out
was that meteors at least 11 million miles out
from the comet's orbit were still moving
approximately parallel thereto and were evi-
dently part of the system. Altogether it
furnished another excellent example of the
connection between comets and certain me-
teor streams. This proof was soon followed
by similar work by Hoffmeister. 20
Since then various members of the Ameri-
can Meteor Society have often observed this
stream, fully confirming the motion. This
has just been most successfully done in 1929
by R. A. Mclntosh 21 of New Zealand. His
work proves the stream still to be most active
though the comet passed perihelion nineteen
years ago. From the United States, these
meteors are seen shortly before dawn in the
southeast. They usually move relatively
slowly, are fairly bright, and often leave
persistent trains.
" Astr. Nach., 191, 251, 1912 and 196, 309, 1913.
" Pop. Astr., 37, 528-34, 1929.
CHAPTER VIII
BIELA'S COMET
"Thereby hangs a tale" SHAKESPEARE
Von Biela, an Austrian officer, after whom
the comet now under consideration was
named, was not its first but its third dis-
coverer. However, circumstances, about to be
mentioned briefly, had prevented any special
attention from being paid to this rather insig-
nificant comet at its two previous appearances.
A Frenchman, Montaigne at Limoges, dis-
covered a comet on March 8, 1772, which he
observed until March 20. Having no suit-
able instruments, his observations were very
approximate. Messier, however, saw it four
more times up to April 3. It was thus ob-
served barely four weeks at this apparition.
Pons, on November 10, 1805, and Bouvard,
on November 16, discovered a comet, whose
coma in two weeks grew to 6' in diameter.
It approached the Earth most nearly on
December 8, when Olbers was able to see it
with the naked eye. Schroter observed it
this same night, finding the comet 30,000 miles
in diameter as seen with naked eye, but only
127
128 COMETS
6000 as seen in his telescope. He measured
the central condensation as 112 miles in
diameter, the actual nucleus as 70 miles.
It was seen this time for four weeks also.
Although elliptical elements were calculated
by several persons, and the identity with the
former comet of 1772 was suspected, for some
reason no one ventured on the comparatively
simple prediction as to when it would return.
Hence, when Von Biela at Josephstadt,
Bohemia, discovered a faint comet on Febru-
ary 27, 1826, neither he nor anyone else sus-
pected at first this was a return of a comet
which had already been observed twice in the
last half century. This time the comet re-
mained visible for eight weeks, and an ellipti-
cal orbit of moderate eccentricity was found
to satisfy the observations. It was now recog-
nized that the comet was the same as the two
mentioned above, but its third discoverer had
the luck to have his name permanently
attached. In table 11 the three orbits are
given along with that for the 1852 return, to
show the changes that occur between returns.
It being established that the comet was one
of short period and that it would evidently
return in 1832, investigations were under-
BIELA'S COMET
129
taken by Damoiseau, Olbers and Santini.
The latter fixed the date of the next peri-
helion at November 27, 1832. It returned
within twelve hours of the exact prediction.
Olbers also called attention to the fact that
the comet would almost cut the Earth's orbit.
This fact, taken up by the papers, was the
TABLE 11
ORBITS OF BIELA'S COMET
PERIHELIO
N PASSAGE
February
Id, 1772
January 1,
1806
April 21,
1826
September
23, 1852
Perihelion distance
Eccentricity
0.9860
0.90314
0.9070
0.74571
0.9024
0.74660
0.8606
75586
Inclination
17 08'
13 37'
13 34'
12 33'
Node
257 16'
251 16'
251 27'
245 61'
Longitude of peri-
helion. .
110 09'
109 38'
109 49'
109 8'
basis for a great "comet scare." Actually the
comet would come to the critical point on the
orbit about a month before the Earth would
reach it, but this little circumstance was either
not understood by or ignored by the sensa-
tional writers of the day. So the Earth was
still about 50 million miles away when the
comet swept by, and of course no harm could
130 COMETS
result. The comet was visible in all eighteen
weeks during this return.
Due to the relative positions of Sun, Earth,
and comet when the latter returned to peri-
helion in 1839 it was always in the twilight
zone, when near enough to be visible, and
hence was not discovered. This perihelion
passage was calculated to have taken place on
July 13, 1839.
February 11, 1846 was calculated by Santini
as the date for the next perihelion. He had
already done other noteworthy work on this
comet's orbit. It was also known that the
comet would be in a most favorable position
for observation, so its return was expected
with eagerness. This was due to the desire
for obtaining observations for checking the
theory of its motion, but no one had any idea
of the surprises in store for the scientific world
that this 1846 return was going to furnish.
At its 1845-46 return Biela's Comet was
first seen by Di Vico at Rome on November
26. It was seen at Berlin by Encke on No-
vember 28, but was extremely faint. It was
seen at Cambridge, England, on December 1
and 3, and at several other places in Europe
during the month. The most remarkable
BIELA'S COMET 131
circumstance attending this return of the
comet was that it was attended by a com-
panion, which was first detected at Yale on
December 29. When discovered this com-
panion was merely a faint nebulous spot,
barely distinguishable; but from that time it
increased in brightness faster than Biela.
On January 13, when it was independently
discovered at Washington, it was estimated
as one-fourth the intensity of Biela. Its
further changes will be shown in the quota-
tions from the actual observers. The com-
panion was constantly growing more distant
from the original comet, being about I 7 on
January 1, 2g' on January 23, and nearly 3' on
February 13.
At Yale the duplication was detected by
Bradley and Herrick on December 29, with
the 5-inch refractor. The position angle of
one to the other was estimated by each
observer drawing diagrams of what he saw.
The Moon was absent and the sky favorable.
On the nights of March 30 and 31, the same
observers estimated the distance to be 16 ; ,
but the companion was barely visible. Brad-
ley was able to measure it as late as April 16.
The following is quoted from the American
132 COMETS
Journal of Science, II, 1,293, 1846: "During
its present return, it has exhibited a very
remarkable appearance. When first observed
through the 5-inch refractor at Yale College,
December 29, 1845, the comet was attended
by a faint nebulous spot preceding, estimated
to be rather more than a minute of space
from its brightest point. The few subse-
quent observations which the clouds and
moonlight permitted here [i.e., Yale], before
the middle of January, showed this secondary
comet to be brighter than the principal, and
slowly departing from it. This surprising
phenomenon was first publicly announced in
this country by Lieutenant Maury, of the
Washington Observatory." Where these ob-
servations were published more fully, if they
ever were, is not known to the writer. Fur-
ther, under modern conditions, priority of
announcement is always counted as priority
of discovery, unless the case is most unusual.
It seems that the importance of the Yale
observations were hardly appreciated by those
who made them.
In England, Professor Challis at Cambridge
on January 15, first saw two comets and then
changed his mind and thought he was wrong.
BIELA'S COMET 133
On January 23 he again saw two, but even
yet was troubled with many misgivings, hav-
ing it evidently in his head that no well-
behaved comet ought to divide in two. On
January 24 he again saw both and apparently
was finally convinced that his eyes did not
deceive him. He then published his observa-
tions. He said that the reason he did not
spend more time confirming matters on
January 15 was that he was anxious to get at
the work on the search for the new planet, i.e.,
Neptune. On the whole, 1846 seems to have
been a year of hard luck for the professor.
He and Airy share the serious blame for los-
ing to England the undisputed priority in
the discovery of Neptune, and here he lost
another opportunity for making a unique
observation.
But of all observers in America and Europe
the one who deserves the greatest credit for
splendid scientific work, of the highest class,
upon this comet, as well as being the (second)
discoverer of its duplication was Matthew
Fontaine Maury, the first director of the
Naval Observatory at Washington, D. C.
Yet one searches almost in vain to find his
name mentioned in American astronomies of
134 COMETS
the present day. How much of this is due to
the deliberate attempt in the years following
the Civil War to belittle his fame, the writer
cannot say. However, he has never received
anything like adequate recognition in this
generation for his superb contributions to
science from the country at large. Maury 1
measured the comet's position on January 12
but did not see the faint companion. This he
discovered on January 13. In all 51 observa-
tions of the comet and 13 of the companion,
on 29 dates up to April 19, were made at the
Naval Observatory of which Maury person-
ally made 49. This series was far the best
secured anywhere in the world.
Let us quote from an original letter that
appeared in part in Monthly Notices, Vol. 7,
90-91: ". . . . [Maury] discovered during his
observations, on January 13th, a nebulous-
looking object, altogether cometary in its
appearance, preceding Biela's Comet by
nine or ten seconds in the lower part of the
field. . . . (January 14) both objects had in-
creased about three minutes in right ascension
since the night before At the time of
1 Astronomical Jr., 1, 135.
BIELA'S COMET 135
the first discovery of their binary character,
No. 2 was | the magnitude, and J the intensity
of its companion. From that time No. 2 in-
creased rapidly both in magnitude and bril-
liancy exhibiting, under the full moon of
February 11, a sharp diamond-like point of
light near its center On February 16th,
as I turned the telescope upon the comet in
the early twilight of the evening, I was sur-
prised to find No. 2 coming into the field
with almost a blaze of whitish light (its color
had been uniformly reddish), while Biela was
barely visible. No. 2 was estimated to be
equal to Biela as to magnitude, but to have
one-third more intensity.
"The next evening was cloudy; and yester-
day being clear, I tested their relative magni-
tudes and brilliancy again in the early twilight.
The contrast was so striking Biela sur-
passed his companion both in magnitude and
intensity, at least twofold Biela was
brighter than it had ever appeared before.
.... No. 2 had assumed its former muddy
appearance, excepting that no bright point of
concentrated light could be seen in its nucleus,
notwithstanding the moon was absent, and
atmospheric conditions favorable.
136 COMETS
"No. 2 appears to have thrown a light arch
of cometary matter from its head over to that
of the other; and their tails stretching off
below in the field, gives these two objects the
singular and beautiful appearance of an
arched way in the heavens, through which
the stars are sometimes seen to pass."
The very complete notes of Maury on the
physical changes are so important, and they
give so much information on the disruptive
forces in action, that they will be quoted at
considerable length. The writer abridges the
parts in parentheses.
"January 14. ... were noticed glimpses of
a tail to each body; (about parallel).
"January 18. Tail of Biela only a few
minutes of arc long, extending to N. E. like a
lancet. (Other's) not so long. Nuclei decidedly
condensed towards the center, but not re-
solvable into points of light except perhaps
Biela's by glimpses.
"January 23. Companion has the appear-
ance of two tails, one nearly parallel to
Biela's, the other reaching over to his nucleus
or rather just to the south of it.
"January 28. Biela exhibited a pointed
BIELA'S COMET 137
nucleus; caught glimpses of a point of con-
densed light in nucleus of companion.
"February 4. A decided stellar nucleus to
each comet, appearing like a sharp point of
light. Tails reaching almost across the field
and nearly parallel.
"February 11. The nucleus of companion
decidedly stellar; that of Biela diffuse, and by
no means as bright.
"February 12. Glimpses of two nuclei in
Biela. (Tail J field.) Tail of companion
nearly parallel. . . . glimpses of another tail
extending towards Biela .... in a sort of
arch. Appearance of two tails to Biela,
second going off in a direction opposite to
companion.
"February 21. Ragged condensation of
light in nucleus of Biela .... tails parallel.
"February 22. A band of nebulous matter,
a little arched, joins the two. Appearance of
a double nucleus about Biela. Has three
tails radiating at angles of 120 to each other.
The tail of Biela extends .... 45'.
"February 26. Biela has a tripod tail ....
the one extending opposite to the companion
very distinct. Confused appearance about
138 COMETS
the nucleus of Biela as though there were
several nuclei.
"March 5. Companion has no apparent
nucleus; is exceedingly faint, and without any
mark of condensation.
"March 8. One of Biela's tails points
directly east the other remains as it was.
"March 10. No stellar nucleus to either
comet. No tail to Biela by the moonlight.
"March 14. Appearance of cometary frag-
ments about Biela. Counted five in this
* ,
position ,
"March 17. Companion is a very diffuse
mass of exceedingly faint nebulous matter.
No appearance of fragments about Biela.
"March 21. Companion very faint,
muddy; a shining point in a dim patch of light
about its nucleus.
"March 30. Saw two tails to Biela as
formerly. Companion not seen." 2
E. Loomis wrote 3 : "From the preceding
observations it will probably not be doubted
that the two bodies had some connection with
each other; that the one which we call Biela
* The notes above are original ones by Maury.
*Silliman'8 Jr., 11, 2, 437, 1846.
BIELA'S COMET 139
was the main body; and that the companion
was formed of matter which proceeded from
Biela. The question then arises, how did the
companion become detached Was there
an internal explosion? Admitting the possi-
bility of an explosion by which a cometary
body might be torn in fragments, the rapid
increase in size and brilliancy of the compan-
ion compared with Biela, from January 13 to
February 16, seems hardly explicable except
upon the supposition of a continued transfer
of matter from one body to the other for an
entire month. It seems then most natural to
suppose that the cause of this continued trans-
fer was the same as that of the first formation
of the companion .... we have observed phe-
nomena in other comets which have some
analogy Halley's comet at its last return
was observed to emit streams of fiery matter,
which exhibited the appearance of sectors of
extreme brilliancy. The matter thus emitted
from the head diffused itself in the direction
opposite to the sun, and formed the tail of the
comet. The attraction of these particles for
each other was scarcely if at all appreciable.
.... According to the rate of separation when
first observed, these two bodies (Biela
140 COMETS
companion) must have been together about
January 1. At this time we may suppose
Biela's comet to have commenced emitting
particles from its head, which uniting by
feeble attraction formed a small nebulous
body. Perhaps when the repulsive force be-
gan to operate, it may have been for a time
resisted by an envelope partaking somewhat
of the character of a solid body; and when
this resistance yielded, a considerable portion
of the main body may have been at once de-
tached by a sort of explosion. A fragment
thus detached might attract to itself at least
a portion of the stream of particles which con-
tinued to be emitted from the main body.
Thus the companion increased at the expense
of Biela until February 16, which was soon
after its perihelion passage. At this time the
attraction of the companion was such that it
could no longer attract to itself the matter
repelled from Biela. It therefore ceased to
grow, and indeed appeared to decline in bril-
liancy, perhaps from the loss of matter which
it emits in the same manner as Biela; although
we have an example in the case of Encke's
comet of a body which habitually becomes less
BIELA'S COMET 141
conspicuous after rather than before perihelion
passage.
"On the whole it must be admitted that the
phenomena of comets are altogether anoma-
lous. The comets of Halley, Encke and Biela,
those of 1824 and 1744, have exhibited phe-
nomena which seem to require us to admit the
existence of matter of a different kind from
anything we witness upon the earth."
Biela was followed elsewhere on this return
by Hartwell in England until March 22, at
Geneva until February 26, and at the Cape of
Good Hope until March 7. As seen, Maury
followed it at Washington with the 9-inch
refractor there until April 19.
On the next return in 1852, the comet was
first discovered by Secchi at Rome. The
companion was in turn discovered by him on
August 26, when he wrote: "I found this
morning the other portion of Biela's comet.
It was very faint, without a nucleus, ....
ovoid form, the apex being turned away from
sun. It followed the other part about 2 m , and
was further south the principal part
of the comet did not continue to appear of the
same figure as at first. It looked quite irregu-
lar and had two very faint streaks; it was more
142 COMETS
luminous in the center, but without any
nucleus."
During this apparition, some observers
measured only one component, some the other,
a few both. On the whole it was found that
they traveled about 1,500,000 miles apart, in
similar orbits, so that it was difficult if not
impossible to tell which of the two was the
original comet of 1846. It was visible five
weeks on this return.
The 1859 return was an unfavorable one for
the comet's discovery, but not that of 1866.
However, diligent search by many observers
failed to detect the comet, nor has it ever been
seen since.
But if Biela's Comet itself was not to re-
appear, the last had not been heard of this
now famous body, though the rest of its
history belongs more to the domain of mete-
oric astronomy. Yet as the debris of comets'
nuclei evidently become meteors, it is clear
that the two types of bodies merge into one
another.
Schiaparelli having in 1866 finally proved
the first case of connection between meteors
and a comet namely, the Perseids and
Tuttle's Comet it became common knowledge
BIELA'S COMET 143
how to proceed in similar cases. Therefore,
the next year both Weiss and d'Arrest, within
a few days of one another, announced that
the meteors which have their radiant in An-
dromeda, and are hence known as Andro-
medes, and which since 1772 had at intervals
furnished moderate showers, moved in the
same orbit as Biela's Comet and were con-
nected therewith.
D'Arrest predicted a shower for December
9, 1878, which did not occur, but Weiss mak-
ing a much more thorough investigation
proved that the node decreased very rapidly
in longitude, and hence any future shower
should come at an earlier date. He thought
that there were good chances for a shower in
1872 or 1879, but that it would occur about
November 28. A. S. Herschel, in England,
asked all observers to be ready in 1872 and
1873 but did not seem to take seriously Weiss's
work proving the regression of the node.
Hence he still expected the meteors early in
December.
Weiss, however, had his prediction brilli-
antly fulfilled for on the night of November 27,
1872, a wonderful display of the Andromedes
or as now usually called the Bielids
144 COMETS
appeared. For instance in Italy, at Monca-
lieri, four observers were able to count 33,400
meteors in the interval from 6 h O m to 12 h 30 m .
In many other places the rate for one observer
was about 10,000 for the same interval. This
display was seen in Europe, for by the time it
was fully dark in America the Earth had
passed out of the dense stream, and therefore
only a moderate number appeared in the
Western Hemisphere. These meteors, since
they overtake the Earth, move with a low
relative velocity. They also do not on the
average appear as bright as those of the
showers which come after midnight.
The appearance of this shower made Klin-
kerfues conclude that, if indeed we were then
passing through the main body of Biela's
Comet, the comet ought to be seen in exactly
the opposite direction from the meteors'
radiant point, as the former went away from
the Earth. So he telegraphed Pogson at
Madras, in India, as follows: "Biela touched
Earth November 27. Search near Theta
Centauri." Pogson at once found a small
cometary looking body in this position. He
saw it on two successive mornings, both times
with a decided nucleus, and on the second with
BIELA'S COMET 145
a tail 8' long. But cloudy weather then came,
and he was unable to find it again once clear
skies reappeared. No one from that day on
has ever seen a cometary body moving in the
orbit of Biela's Comet. Computations by
Newton showed, however, that Biela's Comet
itself must have been, on that date about
200 million miles from the Earth so the object
seen by Pogson could not have been the comet
itself. Newton considered it to be a frag-
ment thrown off long before.
But Biela's history is not yet concluded for,
in 1885 on November 27, another splendid
shower of Bielid meteors appeared. It was
estimated that from one place, if the whole
sky had been watched by observers, about
75,000 per hour would have been seen. For
instance Dcnza in Italy from 6 h O m to 10 h
08 m , having with him from two to four observ-
ers, counted 39,546. The principal shower
did not last over six hours, so the thickness
was not great.
Again on November 23, 1892, a consider-
able shower from the same Bielid radiant was
seen, this time in America. The rate however
was only about 6 per minute, which shows
that it had greatly decreased in intensity.
146 COMETS
On November 24, 1899, the writer, as a boy,
saw the expiring effort (at least to date) when
a shower that attained a rate of 2 per minute
came about 9 p.m. E. S. T. It did not last
over two hours, and was of course seen by
many observers in America. Meteors were
also seen in Europe from the same stream that
year. From then on only a few Bielids have
been seen, in no case enough to dignify by the
name of shower.
Even yet one more proof of its existence
remains, for on the night of November 11,
1928, F. W. Smith, a student at Swarthmore
College, Pa., while guiding on a region in
Andromeda, saw and plotted 11 telescopic
meteors, within a space of 102 minutes. He
turned over his results to the writer, 4 who
worked out an approximate radiant for 10
meteors, and computed a parabolic orbit.
This fits, within reasonable limits, the orbit of
Biela's Comet, showing that small debris from
it was still being met by the Earth. Besides
this unexpected meeting with these telescopic
Bielids, we have had during recent November
observations reported of single bright meteors
whose paths go back to the old radiant.
Astr. Nach., 236, 15-16, 1029.
BIELA'S COMET 147
On the whole, we have here an excellent
example of how a comet is broken up, with
all the successive stages visible. First a comet
of medium size and no special peculiarities
returns several times. Then on one of its
returns it wholly unexpectedly breaks into
two parts, the division being caught even as it
was in process of occurring, and being studied
for weeks by many observers. On the next
return, the companion comets went along
their orbits side by side. On subsequent re-
turns they could no longer be seen with any
available telescope, though unfortunately at
that time there was none of the short focus,
powerful, photographic telescopes available,
with which at present faint objects can often
be detected when no eye can see them.
Then in 1872 the debris from the comet's
orbit met us as a fine meteor shower, repeated
in lessening intensity until 1899 when it died
out. Now we see occasional isolated meteors,
and a small group of telescopic d6bris.
We should indeed add that most of the
meteor showers could not have been from
debris of the comet as it was in 1846, but must
have come from fragments broken from an
original larger comet the parent of both them
148 COMETS
and the final Biela's Comet many years be-
fore. Yet all were moving in approximately
the same orbit, proving their original connec-
tion and common parentage.
Now, so far as we know, both comet and the
condensations that gave the showers of me-
teors are wholly dispersed, and if a shower of
any intensity again comes from this stream
it will cause real surprise. There are well
understood theoretical reasons 6 why this par-
ticular stream was liable to quick disruption,
and would be shorter lived than, for instance,
either the Perseid or Leonid groups.
5 Meteors, 64-73; also 223.
CHAPTER IX
SEVERAL INTERESTING COMETS
"Nor so often did dread comets blaze.' 9
VIRGIL.
The Great Comet of 1858 was discovered
by Donati at Florence on June 2. 1 It has
since borne his name. When discovered it
was 2 astronomical units distant from the
Earth and was a faint nebulous patch without
any remarkable condensation. About the
beginning of September it became visible to
the naked eye. It then had a tail which was
3 long on September 10, 12 long on Septem-
ber 27, and which by October 3 had grown to
36. On October 5 the star Arcturus was
transited by a portion of the tail, quite near
to the nucleus. On October 9, a smaller tail
was sent out, partly coincident with the larger,
but having small brushes projecting from its
convex side. The tail on October 10 was 60
long, 10 wide at end, and sensibly curved, the
convex side being uppermost. The convex
side was well defined, but the lower more
indistinct.
* Man. Not., R. A. S., 19, 141, 1859.
149
150 COMETS
As to its appearance in the telescope, Don-
ati's Comet exhibited very beautiful phe-
nomena, particularly in the development of
concentric envelopes. On September 4, the
nucleus was bright and well-defined; on Sep-
tember 11 the tail was 4.7 long, and the
nucleus was eccentrically placed within the
envelope. On September 16, two diverging
streams of light shot out from the nucleus and,
after separating at the distance of a diameter
from it and going for a short distance towards
the head of the comet, they abruptly turned
backwards and streamed into the tail. M.
Rosa described it as resembling long hair
brushed upward from the forehead and then
allowed to fall on each side of the head.
On September 22 this "parting" gave place
to a fan-like sector, surrounded by a darker
arc, to which succeeded a brighter semicircle
of nebulous light. On September 27 the fan
was more spread out, while on September 30
its axis made a 25 angle with the axis of the
tail. But about this date began also a new
set of phenomena, namely circular or parabolic
concentric envelopes, surrounding the nucleus.
At first three distinct envelopes were visible,
the outer diffused, the second better defined
SEVERAL INTERESTING COMETS 151
and brighter, and the third (separated from
the second by a less luminous interval) in-
creased in brightness towards the nucleus, with
which it was almost confused on the inner
side. On the tail side sectors of 90 were
missing from these envelopes, this space be-
neath the nucleus being very dark. Similar
appearances were observed on October 4. On
October 8 great distortions took place in the
outer envelope. The next night an additional
envelope appeared. These continued until
October 15, when a new set of phenomena in
the shape of "comma-like," curved append-
ages to the nucleus appeared. The latter
seemed to eject a mass of brilliant matter in a
straight line, which was by some other force
violently twisted around. The "commas,"
variously modified, continued until October
22, when their extreme ends had so far turned
back as to nearly complete an elliptically
shaped body. On October 17 the nucleus was
very eccentric with regard to the inner enve-
lope, and somewhat less so with regard to the
outer.
As for the envelopes, seven in all rose from
the nucleus; the highest went out about 18,000
miles, the last about 6000 miles. The nucleus
152 COMETS
showed the usual phenomenon of decreasing
in both linear and angular size near perihelion.
The comet was followed in the Southern
Hemisphere telescopically until March 4,
1859. From a ship 2 in the south Pacific it
was last seen by the naked eye on November
11, 1858, and last with a 2.5-inch glass on
November 16. From this same ship on Octo-
ber 11 the nucleus was estimated as being
equal to a star of the first magnitude, while
the tail was 8 to 9 long. The next night the
nucleus was seen, after sunset, before any
other stars in that region appeared. How-
ever, the comet was seen with the naked eye
elsewhere 3 until December 9; hence it was
visible without optical aid 112 days, with
telescopic aid 275 days. The tail was seen
for 177 days.
The comet passed perihelion on September
29, and was nearest the Earth eleven days
later. Due to its position, near the latter
date, the tail was not appreciably foreshort-
ened, making the comet a magnificent object.
Though its perihelion distance was about 50
million miles, this comet was remarkable for
> Mon. Not., R. A. S., 20, 49, 1859.
Annals H. C. O., Vol. 3.
SEVERAL INTERESTING COMETS 153
the brightness of its nucleus. Its orbit had a
high inclination, 63, and its period according
to different computers was 1879, 2040, and
2138 years respectively. In any case it moves
in a long ellipse, and will certainly return
about the year 4000 A.D. A comet mentioned
by Seneca, and dated in August 146 B.C.,
according to the Chinese, has been thought by
some to be a previous appearance of Donati's
Comet.
COMET 1861
The Great Comet of 1861 was discovered
on May 13 by J. Tebbutt, of New South
Wales, an amateur astronomer who did note-
worthy work. Its perihelion passage took
place on June 11, but it was nearly three
weeks later before it was generally seen in the
Northern Hemisphere.
Indeed it appeared with great suddenness
to American and European observers on June
30 though seen by others the night before.
It was described at New Haven 4 as being
equal to Jupiter in total brightness, but with
an area equal to the full Moon. It had a fine
tail, with sharp, nearly parallel sides the ex-
treme breadth being 10. On July 2, while
4 Am. Jour. Sci., II, 32, 352, 1861.
154 COMETS
the head was not quite so bright, the tail was
90 long. The head was 30' in diameter.
Near the center was a bright nucleus, from
which emanated a luminous sector, with a 90
opening. On July 3, this opening was 136.
Beyond was a dark arch concentric with the
nucleus, then a faint luminous envelope. Next
came a fainter dark arch, and then a second
fainter luminous envelope. The tail was 95
long, its axis deviating 12 from the line of the
Sun. The tail broke off or became faint
about 20 from the nucleus. From this point
it continued as a much fainter milky band,
decreasing gradually in luminosity, its breadth
staying constant, at 1|. The breadth was
3 in brighter portion. The tail was appar-
ently made up of two distinct streams of
luminous matter, differing in width and length.
The northern edges of the two were in the
same line, but the extreme breadth of the
shorter stream was much the greater. Its
southern edge was badly defined, and some-
what concave outward. A very faint diffused
light, rapidly widening, could be traced far
beyond the point where the change in bright-
ness occurred. After July 5 the tail decreased
steadily in both length and brightness.
SEVERAL INTERESTING COMETS 155
According to the Washington observation, 5
already on July 2, the tail had two branches.
The first, slightly curved, was 8 to 10 long.
The other, the wider or eastern branch, was
straight and narrow, 1| wide, and was 80 to
85 long. For the first 10 the tail was
brilliant, then decreased in brightness. The
nucleus appeared like a planetary disc a few
seconds in diameter. Concentric envelopes,
much like those of Donati's Comet were
seen. On July 3 the nucleus was 11" in
diameter and elongated. Three days later
the tail was 25 long, and on July 8 the nu-
cleus was about magnitude 3.
According to Webb iji England, who used a
5|-inch refractor, on June 30 the nucleus was
brighter than Jupiter and 2" in diameter. On
July 4, he states it was only 0.5", which is
quite contradictory to the Washington obser-
vation of the previous night. Webb gives the
coma as 20' on July 4, and 13' on July 15.
On June 30 the comet was golden; on July 10
white to the naked eye, and greenish-blue in
the telescope.
Am. Jour. Sci., II, 32, 305, 1861.
156 COMETS
Chambers 6 states that eleven envelopes
were seen to rise from the comet's head from
July 2 to 19 inclusive, a new one rising every
second day. Their evolution and dispersion
therefore took place very much faster than in
the case of Donati's Comet. Secchi stated
that while the tail and parts of the head near
the nucleus showed strong polarization, the
nucleus itself showed no trace of it. But on
July 3 and following days the nucleus showed
decided indications of polarization.
About 6 p.m. on June 28, the comet's head
was in the plane of the Earth's orbit, about
13 million miles distant on the inside. It
seems quite certain that the Earth passed
through the comet's tail on June 30, as has
been discussed elsewhere (see p. 72). The
difference in date can be easily accounted for
by a very slight curvature of the tail, which is
practically never an exact prolongation of the
radius vector.
HOLMES'S COMET
This comet was discovered on November 6,
1892 by Holmes in London (I). 7 It was then
The Story of the Comets, 157, 1910.
7 Observatory, 16, 142, 1893. Also, Handbuch Astr. & Geoph.,
4, 25, 1893 and 5, 34, 1894.
SEVERAL INTERESTING COMETS 157
5' in diameter and a bright circular mass with-
out nucleus. On November 9 it was a strik-
ingly compact object, with sharply defined
rounded edges. A complete change took
place by November 16 when it was 10.5' in
diameter, and was markedly irregular in
shape. After this a short, faint tail was de-
veloped. On a plate by Barnard on Novem-
ber 10 the comet's diameter was and the
tail 1 long. But during the middle of Novem-
ber the comet's transparency was so great
that faint stars were seen through the densest
parts. It was, however, visible to the naked
eye. Its spectrum was meanwhile purely
continuous, without a trace of bands.
By the middle of December the diffusion
and decadence had continued so far that the
comet was observable with difficulty, even
in large telescopes. This increase in angular
size took place despite the fact that the comet
and Earth were rapidly separating. In Janu-
ary, the comet became brighter and smaller,
but about January 16 it suddenly resolved
itself from a scarcely perceptible mist into a
nebulous star of magnitude 7 or 8, with a
diameter of 30". Coma and tail were en-
tirely missing. On January 18 the stellar
158 COMETS
nucleus was reported by Barnard as being
barely distinguishable and of magnitude 13.
By January 23 it was again 2' in diameter and
the object expanded into a dull nebulous mass,
as before. On February 11, it was fairly
conspicuous in a 10-inch refractor, but it had
no stellar nucleus, instead there was a slightly
condensed region in the head of the comet.
During March it again became very faint, the
last observation being made on March 13.
It was, however, faintly visible until April. So
long as it could be observed the comet had a
continuous spectrum.
In 1899, the comet was found by Perrine on
January 10 ; 8 it was then a round nebulous
object 30" in diameter with a faint central
condensation, the total magnitude being 16.
Its maximum light at this appearance was
only magnitude 14. It was last seen Novem-
ber 6. Nothing happened to recall its strange
performances in 1892. At its return in 1906, it
was again excessively faint. It was not seen
by anyone at the 1913 or any subsequent
return.
Holmes's Comet has a period of 6.9 years.
Handbuch Astr. & Geoph., II, 37, 1900.
SEVERAL INTERESTING COMETS 159
Its orbit has an inclination of 21, while the
least distance to Jupiter's orbit is only 0.4
unit. It is considered to be a member of that
planet's family.
TAYLOR'S COMET
Taylor's Comet, 19150, another member of
Jupiter's family, was seen on February 9,
1916, by Barnard 9 to have two nuclei, 10"
apart, while on former dates of observation
it had shown only one. Both nuclei had short
tails. For a time the north component grew
fainter, the south brighter and more con-
densed. Then the north component bright-
ened and showed strong condensation, while
the south became diffuse and faint; finally
the latter disappeared, leaving the north com-
ponent a strongly condensed comet. Barnard
stated that other comets which showed parti-
tion were: Biela's Comet, Comet 1860 (III),
Great Comet of 1882, Brooks 1889 (V), Swift
1899, Kopff 1906, and Halley's Comet in May,
1910.
Mon. Not., R. A. S., 77, 355, 1917.
CHAPTER X
MOREHOUSE'S COMET
Comet 1908c, better known after the name
of its discoverer as Morehouse's Comet, was
discovered on September 1, 1908. Its orbit
had an inclination of 140, hence its motion
was retrograde, and though it was observed
for more than six months there was no appre-
ciable deviation from parabolic motion. It
passed perihelion on Christmas Day, hence
it was well observed both as it approached the
Sun and regressed. Its considerable peri-
helion distance of 0.94 kept it from approach-
ing the Sun's vicinity, where violent changes
in comets would be most probably expected.
It was discovered by photography, but during
part of its career was a naked-eye object,
though never a brilliant one by any means,
its maximum brightness being 5 magnitude.
It was circumpolar for some weeks in the
fall of 1908, and indeed traveled across the
heavens before it disappeared. These favor-
able circumstances, coupled with the extra-
ordinary phenomena which it exhibited, made
160
MOREHOUSE'S COMET 161
it one of the most observed and best known of
comets.
Baldet 1 remarks that this comet might be
called a blue comet, in contrast with Daniel's
Comet of 1907 a yellow one. The latter
was bright to the eye, but Morehouse's Comet
could be photographed with relatively short
exposures, and splendid results obtained.
Neither the Swan nor cyanogen spectra ex-
tended, however, into the tail. Nor was there
the slightest trace of hydrogen, despite an
erroneous identification of that element. 2
The absence of the usual continuous spectrum
in certain regions was also remarkable.
Further the knots or luminous clouds thrown
out by the nucleus into the tail had the same
spectral constitution as the latter and not that
of the head from which they emanated.
Neither the Swan nor cyanogen spectra could
ever be detected in the spectra of these knots.
Baldet lists 82 radiations that he was able to
detect in the spectra of this comet. Another
interesting thing about the comet's spectrum
was the doubling of a number of lines, the
components being about 20 angstroms apart.
1 Page. 24, loc. cit.
* A sir. Nach., 179, 193, 1908.
162 COMETS
This phenomenon was seen certainly as late
as March 21, 1909.
Direct photographs recorded the strangest
and most violent changes in the tail. The
most remarkable occurred on September 30-
October 1 and on October 15. The first
transformation began between September 29
and 30 and ended the next day. The following
account is abstracted from Prof. E. E. Barn-
ard's article. 8 On September 30 a violent
change was taking place through the night.
On the previous night a plate already showed
a disturbed condition in the comet. On the
first plate of September 30, the head was
small, and from this a thick tail ran out in a
straggling manner with a fainter sheeting of
matter having a sharp edge on the south
side. On the next plate the whole tail had
moved out bodily and was connected with the
head by a very narrow tapering neck. The
tail was wide and large, widening out very
greatly as it left the head, being curved and
brighter on the northern side. Fluffy masses
projected from this north side. On the third
plate, the tail had tapered down to a very
8 Astroph. Jr., 28, 292, 1908.
MOREHOUSE'S COMET 163
narrow connection with the head which was
almost starlike. The fluffy masses had be-
come a large projection. The tail appeared
cyclonic. Doubtless an hour or so later the
whole tail had become disconnected from the
head, as the separation is essentially shown on
the last plate. A similar separation was
shown on a plate of September 20. The first
plate on October 1 showed what was evidently
the great mass of matter which formed the
tail on September 30, now about 2 out from
the head. The tail was 8 long. The outer
6 of this was made up of an irregular, long,
straggling mass which had a tendency to
spread northward. This great mass was
apparently attached to the head by a slender
thread. There was a narrow, short ray from
the head, at a 45 angle on the south side.
On the second plate two rays connect with
the great mass, one of which after running
parallel with the main one for a degree, bends
in and joins it, making only one ray that
reaches the head. On the third plate a dif-
fused ray had shot out for 1 on north, while
there were several streamers connecting the
mass with the head. On the fourth plate, the
new ray had merged. On the fifth, the new
164 COMETS
rays curved northward and joined the system
of rays from the head at a distance of 2. The
outward end of a ray system had become dis-
connected from the great mass, which had
become square in form and now sharply de-
fined at the end nearer the head. On the sixth
plate the separation was complete and the end
of the great mass more pointed.
On October 2 the comet had a changing
system of broad curving streamers spreading
out at wide angles. On October 3 the tail
consisted of a widely diverging skeleton
framework changing rather slowly. On Octo-
ber 4 the tail had become a normal one again.
On October 15 the first plate showed a
straight narrow tail long, with short rays
out from the head at angles of 30 and 45,
on either side. At the end of the J tail began
a most unusual tail which was twisted and
clouded at the beginning and which streamed
out irregularly, bending northward with ir-
regular outline for 7 to 8 to the edge of the
plate. On the first plate the straight tail
joined the south portion of this twisted mass,
and in the last picture of this date it made a
junction further north at about the middle of
the mass which was J broad where it began.
MOREHOUSE'S COMET 165
These masses were very dense and, from the
south part, narrow streamers ran out parallel
with the short tail for about 2. In the pho-
tographs of October 14 there were no indica-
tions of this disturbance. Remnants of these
cloud masses were shown much farther out on
plates of October 16 and 17. However 4 on
October 15 plates were taken at Geneva and
Juvisy, 8 hours and 7 hours respectively
earlier than Barnard's first plate of that date.
On the Geneva plate about 20' from the head
there was a strong bend in the tail but no
masses. On the Juvisy plate this bend was
stronger and more suggestive of the appear-
ance later shown on Barnard's plate. The
latter therefore concluded that the masses
were not thrown off as such from the comet,
but had their origin in a disruption of the tail
which must have occurred just previous to
7 h G.M.T. Barnard said this concurred
with his idea that comet tails encountered
some sort of disturbing medium in space.
The variations in the appearance of the
comet were the most rapid ever observed.
Many times photographs, taken at one day's
interval, appeared so different one would not
4 Aatroph. Jr., 29, 70, 1909.
166 COMETS
know that they belonged to the same object.
One fact was clearly established, namely, that
the formation of the tail was intermittent and
not continuous. The formation of parabolic
envelopes on the side towards the Sun was
most striking. Eddington 6 says that the en-
velopes in the Greenwich photographs were for
the most part sharply defined, fine curves,
differing markedly from similar appearances
in other comets. He considered that each
envelope corresponded to a distinct explosion,
while in other comets the explosions were so
numerous that only a general effect was
noticed. Here the individual envelopes were
all most transitory, and immediately after
their formation began to collapse and shrink.
None expanded. After three or four hours an
envelope had completely degenerated,' the
part between the nucleus and the Sun being
lost in fresh material rushing out, which would
form new envelopes, that behind mingling with
the tail. Envelopes on contracting were more
sharply defined. The size therefore seemed
to depend on the age. He concluded that, if
we accept the simple theory of the formation of
envelopes, i.e., that they are formed by jets
6 Mon. Not., R. A. S., 70, 444, 1910.
MOREHOUSE'S COMET 167
from the nucleus, then the repulsive forces
must be from 10 to 100 times as great as those
that act on particles in the tail.
Barnard noted that parts of the tail suddenly
brightened up where apparently no material
previously existed, and with no visible supply
coming from the nucleus. An explanation
was that such knots or condensations in the
tail were due to the superposition of detached
rays or band crossing one another, giving
rise to increased brilliancy at their intersection
points, and shifting with the relative positions
of the rays.
In the Southern Hemisphere, the comet
could be seen by the naked eye as late as
March 13, 1909. On March 3 the tail was
3 long, and on March 13 was 1 long and
nearly severed from the head. Another ob-
server with a 9-inch telescope on March 17
saw the comet as very bright, with head large
and condensed in center surrounded by a faint
sheen with fan-like streaks. The tail was
straight, 2 long, and fairly well defined. The
comet was traced in a 10-inch reflector until
April 15, when the coma was broad and
diffuse. The tail was 20' long, broad and
very faint. With the same instrument it
could not be seen on May 10.
CHAPTER XI
PONS-WlNNECKE's COMET
This comet was discovered by Pons on
June 12, 1819, and rediscovered by Winnecke
in 1858. It has been observed at the follow-
ing returns: 1819, 1858, 1869, 1875, 1886,
1892, 1898, 1909, 1915, 1921 and 1927. The
comet's orbit has a small inclination, belongs
to Jupiter's family, and undergoes very great
perturbations from Jupiter. It also comes
near the Earth's orbit. Table 12 illustrates
the great changes in its elements on several of
its returns.
It will be noticed that its period is almost
exactly half that of Jupiter's (11.86 years),
which has made investigations of its orbit of
particular interest. This has, indeed, been
the subject of a very great deal of study on
the part of many astronomers, and will doubt-
less continue to be for many future returns.
The comet has never shown a tail, a circum-
stance due almost surely to the fact that all
tail-forming material has long since been ex-
pelled. This is a phenomenon quite general
with comets of Jupiter's family, which per-
168
PONS-WINNECKE'S COMET
169
force approach the Sun at such short intervals
of time.
As the comet is a relatively inconspicuous
object, and presents no special features of
interest other than changes in the orbit at
different returns, we will pass over accounts
of its earlier appearances, and begin with that^
TABLE 12
CHANGES IN THE ELEMENTS OF THE PONS-WINNECKE COMET
YEAR
a
e
i
IT
Q
P
1819
3.160
0.756
0.774
10 43'
274 41'
113 11'
5.62y
1858
3.137
0.755
0.769
10 48
275 39
113 12
5.56
1898
3 242
0.715
924
17 00
274 14
100 53
5.84
1909
3.262
702
0.973
18 17
271 37
99 21
5.88
1915
2 825
701
0.972
18 18
271 43
99 23
5 87
1921
3 297
684
1 041
18 55
268 24
98 06
5.99
1927
3.305
0.686
1.039
18 56
268 32
98 09
6.01
of 1915. The reason is that in June, 1916,
when the Earth was nearest the comet's orbit,
though the latter had passed by nine months
before, a meteoric stream of some intensity
was met, which traveled in almost the same
orbit as did the comet. This circumstance
brought greater attention to the comet, which
has not diminished at subsequent returns.
Something further will be said of the accom-
panying meteors at the end of the chapter.
170 COMETS
In 1915 the comet was detected by photog-
raphy at Bergedorf on April 4, by Thiele; its
magnitude was 16. Passing perihelion on
September 1, it came nearest the Earth on
September 24, but it was a faint object, being
more than a unit distant. At Johannesburg
on November 8, it was described as 20" in
diameter and of magnitude 12.
In 1921, Barnard at Yerkes found it on
April 10, when its magnitude was 12. It
brightened rapidly until it was 6.5 magnitude
in June. On this return, for the first recorded
time, its perihelion fell outside the Earth's
orbit (see table 12). This was due to enor-
mous perturbations by Jupiter which increased
the perihelion distance about 5 million miles
between the 1916 and 1921 returns.
On the 1927 1 return the comet was detected
by van Biesbroeck at Yerkes Observatory on
March 3, though later it was found on a
Greenwich plate of February 25. Its magni-
tude was 15 or 16. By April 9 the comet was
about 9| magnitude, diameter 6' or 7'. There
was a central condensation, which was not
stellar, of magnitude 11. On May 2 it was
1 Obs., 50, 127, 1927.
PONS-WINNECKE'S COMET 171
circular and 5' in diameter. The central 15"
was much brighter. The nearly stellar nu-
cleus was magnitude 12-13, the total light
equaled a 9 or 10 magnitude star. But on
May 3 it was down to magnitude 11. On
May 20 it was about 9' in diameter, with a
nucleus of magnitude 11. The inner coma
was brightest in the second quadrant suggest-
ing a broad streamer emitted in that direction
by the nucleus. No tail was seen, however.
During June, when the comet came nearest
the Earth, its total light equaled magnitude
4. The nucleus was sharp and well defined.
Steavenson, using the Greenwich 28-inch, a
few days before the date of nearest approach,
observed the nucleus as practically stellar
and about I" in diameter. The comet on
June 26.7 came within 3.6 million miles of the
Earth, a record broken only by Lexcll's Comet
in 1770, 2 which came to within 1.5 million
miles, when its apparent diameter was 2.
During this period 3 Baldet at Meudon esti-
mated the diameter of the nucleus as less than
1 km., and Slipher at Flagstaff as about 2
miles. The comet, during June, traveled
2 Chambers, 39, 87.
8 P. A., 35, 412, 1927.
172 COMETS
down the Milky Way, from north to south,
and in general appeared as a round hazy spot
about 1 in diameter. The nucleus equaled a
9 magnitude star. From it emanated a nar-
row pencil of light towards the Sun, which
gradually spread out fanwise. The comet
resembled in brightness and appearance the
Nebula in Andromeda as seen by the naked
eye. During the latter part of June it moved
many degrees per day traveling 106 across
the sky from June 10 to July 2, By the end
of the month it had gone too far south for
successful work by observers in the Northern
Hemisphere. The comet was, however, ob-
served at Johannesburg until December.
Objective prism plates at Yerkes Observa-
tory showed main condensations due to C IV,
C + H, and Cy IV, though the details were
diffuse. At the Lick Observatory 4 on June
23, a one-prism spectrograph attached to the
36-inch refra.ctor secured a strong spectrum
of the comet, with an exposure time of 5.4
hours. The comet that night had a very
sharp stellar nucleus of 9.5 visual magnitude.
Extending from it east of north was a bright
Pub. A. S. P., 39, 222, 1929.
PONS-WINNECKE'S COMET 173
fan-shaped jet or streak. The spectrum,
extending from \3900 to \5000, appeared to
be the usual solar spectrum with the carbon
and cyanogen bands superimposed. Both
the carbon and cyanogen bands are much
fainter and the Fraunhofer spectrum of the
nucleus is much stronger than in the spectra
of bright comets. Such a spectrum is similar
to that of other short period comets, and is in
harmony with the opinion that such bodies
have largely lost their gaseous material, due
to numerous returns to perihelion.
Slipher at Flagstaff reported that the
"spray of light" appeared to be more emissive
than any other portion of the comet. The
nucleus appeared sharp and only 2 or 3 miles
in diameter, but on one or two occasions faint
secondary condensations were seen. These
latter led others to question whether the
nucleus represented the exact center of mass
of the body. Thiele suggested that perhaps
a rotation of the nucleus around this hypo-
thetical center of mass might explain some of
the anomalies and difficulties found in com-
puting an exact orbit from the observations.
Baldet 6 using the great Meudon refractor
" Bui. Astr. Soc. de France, 41, 401, 1927.
174 COMETS
of 0.83 meter aperture, made extensive obser-
vations when the comet was nearest. He
states that the nucleus was a stellar point,
almost at the limit of visibility, perceptible
only with high power and in good seeing. It
was surrounded by a circular nebulosity 2" to
3" in diameter (which would correspond then
to from 60 to 90 km.) which was sharply de-
tached from the coma in general. The nu-
cleus proper must have been less than 5 km.
across. In his opinion the nucleus could only
have been one solid body, for he says that a
swarm of corpuscles submitted to the various
perturbations and the expansion of the in-
cluded gases would have been dispersed. He
found the nucleus of magnitude 13, but it
with the gas around it which would all
appear as the nucleus in a smaller telescope
to be of magnitude 10. From photometric
considerations he derived 400 meters as the
probable diameter of the solid nucleus. The
total diameter of the coma, determined with
the naked eye, was about 300,000 km. Pho-
tographs did not show so large a value. The
brightness of the whole was 4 or 4 magnitude.
He adds that the attraction of the nucleus
upon the surrounding gases could not hold
PONS-WINNECKE'S COMET 175
these, as a planet does its atmosphere.
Therefore this gaseous envelope must be con-
stantly lost and as constantly renewed. This
seems to be done by jets, great and small,
coming out of the nucleus and oriented toward
the Sun.
A glance at the table of elements shows that
at the 1909 return the perihelion distance had
already approached unity. So far as the
writer knows, though he has made no search
to check his memory, no meteors were seen
in 1908-1910 which were then considered to
be connected with Pons-Winnecke's Comet.
However, about fifteen years later when the
fact of the great fall of meteorites in Siberia,
which took place on June 30, 1\908, was
authenticated, Meltzev calculated that these
meteorites probably followed the orbit of the
comet and were connected therewith (see p.
203).
However, the discovery of a meteor shower
in connection with this comet was made in
June, 1916. Quite a bright shower was seen
by W. F. Denning and others on June 28, the
meteors coming for a time at the rate of one
every minute or two. Previously, late in
May and early in June, numerous meteors
176 COMETS
were seen in America. Their radiants gave
parabolic orbits 6 which fitted the comet's
orbit approximately. The radiant of the
bright shower on June 28.5 G. M. T. also coin-
cided with that of meteors in the comet's orbit.
The connection of these meteors with the
comet was thus plain.
In 1921, as the comet's perihelion had mean-
time continued to move outward, there was
much uncertainty as to whether the accom-
panying meteor stream had passed on too far
to be cut by the Earth's orbit. Nevertheless
a shower was carefully looked for, with en-
tirely negative results in both Europe and
America. Only from Japan 7 came the ac-
count of a strong shower, made up almost
wholly of meteors below magnitude 5. No-
where else was this confirmed.
In 1927, the comet itself passing so very
near the Earth, there was a much better
chance. Observers everywhere were asked
to be on the lookout. But again general
results were disappointing. Indeed reports
were so contradictory that it is difficult to
know what did happen. From Russian Turk-
Mon. Not., R. A. S., 77, 71, 1916.
7 Mem. Kyoto Impr. Univ., V, 5, 1922.
PONS-WINNECKE'S COMET 177
estan came an account 8 of a strong shower,
again of faint meteors. In America in iso-
lated localities many meteors were reported,
but places a few hundred miles away saw
almost none. It is, however, certain that
many fireballs did come at the critical epoch,
some of which doubtless belonged to the
comet's debris. One that appeared over
Tennessee on June 27 at 14 h 22 m 37- C. S. T.
had its orbit computed by the writer. 9 The
elements, which were rough, rather resembled
those of the comet's orbit. A wonderfully
bright fireball 10 photographed in Manchuria
by Yamamato on June 29, 1927 at 15 h 51 m
G. M. T., was considered to be a member of
the comet's family.
However, from what we know of similar
meteor showers the writer is forced regretfully
to express the opinion that the chances are
against its reappearance in strength, rather
than in its favor. This is due to the fact that
already the perihelion point has been shifted
beyond our orbit. Unless perturbations in
future shift it back, we will probably see fewer
and fewer of these meteors in coming years.
A. N. 9 232, 283, 1928.
P. A., 37, 343, 1929.
" P. A., 36, 496, 1928.
CHAPTER XII
COMET 1910a
"Cosi Beatrice: e quelle anime liete,
Si fgro spere sopra fissi poli,
Fiammando forte a guisa di comete."
DANTE.
This comet was discovered about the middle
of January, in South Africa. It passed peri-
helion on January 17. After this it rapidly
moved northward, and at first it was hoped
that we would have the opportunity of seeing
a really great and brilliant comet. This hope
was not fulfilled however, though the comet
for a few days was a bright and conspicuous
object.
At the Lick Observatory, where the writer
then was, a search was made for the body on
the afternoon of January 17 1 but it was not
found. Next day it was plainly seen about
11 a.m., 4 east of the Sun and much brighter
than Venus, which was easily visible 30 away.
The comet had a short tail about 1 long,
which was also easily seen with the unaided
eye. Visual observations by Wright 2 showed
1 Pub. A. S. P., 22, 29, 1910.
* Lick Obs. Bui., 174, 1910.
178
COMET 1910a 179
the spectrum of the nucleus to be continuous,
crossed by the bright D lines, extending 12"
to 14" out into the coma. With a 6-inch
telescope, the nucleus appeared very small
and bright, with a sharply defined edge, with
no gradual shading off into the coma. Stream-
ing back from and surrounding it were nebu-
lous envelopes, traceable for . Next day,
i.e., January 19, efforts to locate the comet in
the daylight sky were fruitless and, at dark,
clouds came up that lasted until January 26.
Plates were taken by Paul W. Merrill and
the writer with the Crocker 6-inch telescope
on January 26-30 inclusive and on February
1. The nucleus, as seen on these plates, on
January 26 was sharp, on January 27 it was
elongated N and S, and on January 28 and
29 round. As for the tail it was trifurcated
on January 26 to within 6 from the nucleus.
On another plate, with the nucleus off the
plate, 14 of it could be traced, with a great
fork at 13 from the nucleus. The trifur-
cated tail could be traced 8 on one plate on
January 27, with ll m exposure. The great
fork plainly shows on another plate, having
14 of the tail on it. On January 28 the
southern branch was relatively stronger, 5
180 COMETS
of the tail being photographed. On January
29 the great fork still showed plainly, 14 of
the tail being on the plate. On January 30,
with hazy sky, the tail was faint and short.
On February 1, the tail was 2 long, with the
southern branch relatively stronger. Nar-
row, sharp streamers and bright knots or con-
densations nowhere appeared. The southern
branch, which made a smaller angle with the
radius vector than the others, did not extend
so far from the nucleus in proportion to its
strength . It, however, while diminishing daily
in absolute intensity, gained in relative in-
tensity.
As to the visual observations on these same
nights, the tail was seen to be from 15 to 30
long. On January 27, Aitken described it
as in "the form of a long feathery plume, curv-
ing slightly toward the south from a vertical
direction, until it reached a point about 15
from the head; then the tail forked, and the
curvature toward the south of the main part
became, rather abruptly, very much moie
pronounced. This branch could be traced at
least 15 further. The northern fork could
only be traced 2 or 3."
Curtis photographed the comet with the 37-
COMET 1910a 181
inch Crossley reflector on February 1, 2 and
5. On the plate of February 1, the head
proper was very bright, round, without appar-
ent nucleus, and slightly over I' in diameter.
Numerous streamers radiated from the head,
but no knots or condensations were shown.
Albrecht with the 36-inch refractor and a
grating spectrograph, using sodium flame as a
comparison spectrum, photographed the T>i
and D 2 lines. The second was bright, on the
plate. The DI line was faint. Measures of
D 2 gave R. V. = 63.3 km/sec, while the com-
puted R. V. was 62.7 km/sec. The weighted
mean of D t and D 2 was 66.1 km /sec.
Wright observed the spectrum visually on
January 26, finding "some continuous spec-
trum and a number of bands, probably the
regular cometary bands, also a bright line in
the orange, undoubtedly the D lines, blended."
A plate taken at same time showed faintly the
bands and D line. On January 27 he says:
"the three bands present, also D line. There
is a pronounced brightening just to the red of
D." On January 30, the D lines were no
longer visible, but the brightening in the red
was faintly seen.
The comet, having switched to the morning
182 COMETS
sky, was seen during the early part of March
being of magnitude 8 on March 12. Dur-
ing the first few days of its appearance as an
evening object, several observers had esti-
mated its head as being of the first magnitude.
Barnard 8 with the 40-inch Yerkes refractor
measured it up to June 12. He saw the comet
on April 12 as "faint, feebly condensed; 13th
or 14th magnitude, rather large; no nucleus or
elongation." On June 7 : "There is a slightly
brighter portion 5" in diameter. The whole
is perhaps 15" in diameter and the brightest
part 16th magnitude." On June 12: "Very
faint. It diffuses over perhaps J' or more.
There is a more condensed portion 5" diameter
and 16th magnitude."
Aitken on April 16, with 36-inch Lick re-
fractor, saw the comet as of 12 to 12J magni-
tude, with feeble condensation and indefinite
boundary.
Baldet found that the spectrum of the comet
was very unique. The tail gave, for a length
of 8, a continuous spectrum. But the disper-
sion was small, 7 mm. from \4740 to \3880.
This was not at all like the spectra of the tails
*A8troph. Jr. ,28, 137, 1910.
COMET 1910a 183
of Daniel's Comet and Morehouse's Comet, at
least up to 5 of the nucleus. Very near this
latter in three places were feeble prolongations
which perhaps could be identified with it.
The nucleus gave an intense continuous spec-
trum upon which were detached images of the
head, of which the most brilliant was the blue
band of the Swan spectrum. Baldet had
plates taken on January 22, 29, and 30; only
for the middle date was the plate satisfactory,
due to poor observing conditions.
Sodium rays were seen from the first day of
discovery; then they rapidly diminished in
intensity while the Swan spectrum became
more and more brilliant. The variations
resembled those of the Great Comet 1882.
Newall with a direct vision spectroscope
saw the comet almost entirely in sodium light
on January 22. "Then as the comet left peri-
helion, which had taken place on January 17,
the sodium image became both more feeble
and less extended, up to when it was emitted
only by the nucleus. The usual green image
was emitted only by the nucleus also. On
January 22 at Meudon with an objective
prism the sodium image was photographed to
within 20' from the nucleus. Here also was
184 COMETS
obtained an image in the red. 4 From the side
of the extreme red, the continuous spectrum
showed a very short reinforcement from \620
to X700, which prolongs into the tail for 10',
and which perhaps belongs to a group of in-
tense bands common to the nucleus and to the
tail, and not noticed in other comets."
As to the origin of these it is uncertain.
They might be due to the red spectrum of
cyanogen.
There was great trouble in determining the
orbit of this comet. The several preliminary
orbits published differed widely from one
another. As an example, the first three gave
the inclination as 62, 85, and 57 respectively.
A correct orbit finally gave it as 139, entirely
reversing even the direction of motion! Its
perihelion distance was q = 0.13. No de-
cided deviation from a parabola could be
found.
The reason that the comet was not dis-
covered until it was a brilliant object was, as
usual, that in coming toward the Sun it had
kept, as seen from the Earth, in the very near
neighborhood of the former body.
4 C. R., 150, 254, 1910.
CHAPTER XIII
COMETS AND METEOR STREAMS
"And the stars shall fall from heaven even as a fig
tree casteth her untimely fruit when shaken by a
mighty wind."
Having outlined the various theories of the
behavior of comets which are typical or appear
worthy of mention, we shall now give the
connections that have been proved between
comets and meteor streams, and between the
orbits of comets and certain other bodies.
The writer believes that only in view of these
connections and analogies can we hope to
secure a comprehensive view of the questions
at issue. All references here to mere opinions
will be omitted and the discussion will be
limited to facts which are accepted generally.
In 1866 Schiaparelli 1 proved that the Per-
seid meteors, which now come to a maximum
about August 11 each year, move in the
same orbit as Tuttle's Comet (Comet 1862
(III)). In 1867 Peters similarly announced
that the Leonids, the meteors which furnish
1 Meteors, Chapter IV.
185
186 COMETS
the great showers every thirty-three years,
follow the same orbit as Tempers Comet.
Shortly afterwards similar connections were
proved for Biela's Comet and the Andro-
medes or Bielid meteors; and for the April
Lyrids and Comet 1861 (I). Since that time
it has been proved that the May Aquarids
move in nearly the same orbit as Halley's
Comet (p. 125), and a shower of meteors has
been connected with Pons-Winnecke's Comet
(p. 175). In addition a few other minor
showers have been announced as following in
the same paths as various comets, but so far
these cases rest on slender evidence, and are
scarcely considered as proven.
These facts lead at once to the conclusion
that there must be a very close connection
between certain comets and meteor streams
which are either annual or at least periodic in
character. Being the one best known, the
Leonid system will be discussed here as an
example. 1
A search of old records has shown that
brilliant meteoric showers have been coming
ever since the year 902 A.D., at least, at times
of the year which correspond now to the
middle of November. The interval between
THE GREAT BOLIDE OF AUGUST 13, 1928
Photographed by M. de Kerolyr at Astrophysical Station of Haute-
COMETS AND METEOR STREAMS 187
successive showers was usually thirty-three
years though sometimes the showers came
more than one year in succession. As stated,
when the orbit of these meteors was calcu-
lated, it proved to be the same as that of
TempePs Comet.
In records previous to 1800, there is little
or no mention of meteors on nights preceding
or following the one on which the great shower
appeared. We are hence unable to make any
deductions as to the total width of the
Leonid stream at these returns. Attention
had been sufficiently focused upon such mat-
ters by about 1860 for records to be made on
several nights. We thus know that some
Leonids appeared at least a day earlier or
later than the maximum date. By the 1899
return the writer had begun personal work,
so that he can state on his own authority that
from 1900 on one might see Leonids on as
many as four successive dates at least. Care-
ful observations in 1928 and 1929 show that
Leonids may be seen over an interval of nine
days. But it is impossible to conclude with
certainty from this that the Leonid stream has
doubled its width in the past half century or
so. It probably merely means that there are
188 COMETS
more observers, who work on more nights than
formerly.
Be this as it may, we can picture the Leonid
stream about as follows. The major axis of
the orbit is 9.5 x 10 8 miles long, or 20.68
astronomical units. Its eccentricity is 0.90,
so the orbit's major axis is 2.29 times as long
as its greatest width, i.e., its minor axis. Let
us now take the inner tube of an automobile
tire and bend it ijnto such an ellipse. Put the
Sun at one focus, and have the diameter of the
tube equal to the distance that the Earth
goes in passing through the stream each No-
vember, making due allowance for the cross-
ing not being at right angles. Now consider
the tube sparsely filled with meteors, all mov-
ing in the same direction, and taking 33
years to make a complete journey from peri-
helion to perihelion again. But for a part of
this stream of such length that it takes three
years to pass the Earth's orbit, as it comes to-
wards the Sun, the meteors are much more
closely packed, and in the very middle of
this "three-year section" packed very closely
indeed.
The Earth passes through this tube about
the middle of each November, usually meeting
COMETS AND METEOR STREAMS 189
only the sparsely scattered Leonids. But at
33- or 34-year intervals it goes through the
dense part and for three or four years we have
much finer showers. And if, as in 1799 and
1833, the Earth happens to hit the very dense
part the "gem of the ring" as it is sometimes
called then we have a grand meteoric shower.
Nevertheless we must confess that on some
of the proper dates, as for instance in 1899, no
rich shower occurred. We can only infer the
reasons for the various blank dates during the
past one thousand years, but in the 1899 case
the cause is well understood. This is scarcely
the place to enter into technicalities about
meteor streams, so all that need be said is
that, as the group which should have met us
in 1899 was on its way Jupiter happened to
be in that part of its orbit very near the
meteors' orbit. Its great perturbations
sufficed to switch the main stream aside
enough to miss the Earth, as it passed us in
1899. But Jupiter having moved on before
the 1901 group passed his orbit, these Leonids
gave us quite a respectable, if not brilliant,
shower in November of that year.
If it is asked whether another grand shower
will come in 1932, 1933 or 1934, or for that
190 COMETS
matter in each of these years, as yet no definite
answer can be given. It all depends upon
the perturbations suffered by the main groups
of Leonids in the past thirty-three years. No
one appears to have attempted to calculate
these perturbations, which present a most
troublesome problem. So far as an off-hand
opinion goes, the meteor stream meantime may
have been switched either toward or from us.
In the first case we should see a really fine
shower, in the latter next to nothing. We
simply do not know yet what to expect.
Coming now to similarities between the
orbits of comets and those of other bodies, let
us first discuss the asteroids. These are nu-
merous small bodies, about 1500 being already
known, nearly all of whose orbits lie between
those of Mars and Jupiter. They vary in size
from Ceres, whose diameter is 480 miles, to
the smallest so far discovered which may be
of the order of 5 miles or less. Smaller ones
certainly exist, but are too faint for ready
discovery. Doubtless there is no limit to
their size, which may range on down to mere
particles of matter like meteors.
Leaving out a few exceptional cases, they
do not seem to penetrate inside the orbit of
COMETS AND METEOR STREAMS
191
Mars or go appreciably outside that of Jupi-
ter. But to discover objects 50 miles in
diameter or less would be almost hopeless if
they were outside Jupiter's orbit. Few of
the known asteroids are so large. Hence
there is neither proof nor good reason to as-
sume that asteroids do not exist between
TABLE 13
ASTEROIDS WITH EXCEPTIONAL ORBITS
719
ALBERT
887
ALINDA
944
HIDALGO
945
1924TD
1927BD
a
2.58
2 53
5.71
2.6
2.66
10.66
e
0.54
53
0.65
16
54
0.76
i
10.8
9.0
43 1
33.0
20 1
6.0
P
4.12y
4.2y
13. 7y
4.3y
4.35y
34. 8y
Jupiter's and Saturn's orbits, or perhaps even
farther out from the Sun. 2
All of these objects so far discovered move
with direct motion, and many of them in
orbits of moderate eccentricity and inclina-
tion. But there are some most exceptional
cases, of which a few wiU be quoted (see table
13). If we compare these orbital elements
8 For a modifying opinion see, however, E. W. Brown,
Resonance in the Solar System, Bui. Am. Math. Soc., May-
June, 1928.
192 COMETS
with those of the short period comets we find
many striking resemblances. In fact many
of these comets' orbits have much less inclina-
tion and less eccentricity than have the aster-
oid orbits mentioned. This would mean
that such an asteroid, if it showed a coma or
tail, would be classed most certainly as a
comet. Where then is the dividing line?
However, the question can be more properly
discussed in all its various phases in the
next chapter, so only this preliminary sketch
will be given here.
The mass of literature on comets is so great,
and in so many languages that the writer is
unable to state who was the first seriously to
suggest possible connections between aster-
oids and comets. The idea was, however,
somewhat fully outlined by him in 1924. 8
8 Meteors, 264-272.
CHAPTER XIV
COLLISIONS OF COMETS WITH THE EARTH
"And there fell a great star from heaven, burning as it
were a lamp. And it fell upon a third part of the
rivers "
One day a few thousand years ago, a fright-
ful apparition appeared in the heavens over
the western part of what is now the United
States. A tremendous blazing mass shot
downward through the sky with terrific speed
from north to south. As it came lower it
was surrounded by smoke, and it looked
something like the blackest of storm clouds.
The roar was terrific and the air waves set up
by its passage were felt far and near. Finally,
with a crash that exceeds description, it
struck a semi-desert part of what is now
Arizona. Instantly into the air arose a
gigantic column of debris and rock dust, while
on the borders of the immense crater pit
formed by the impact were deposited masses
of shattered rock of all sizes falling back from
the sky. A violent earthquake, which how-
ever was over very quickly, spread further
consternation among all the tribes on or near
193
194 COMETS
the Pacific Coast. Had there been forests
anywhere within miles of the crater the side-
ways rush of the super-heated and compressed
air would have laid them level with the
ground, as indeed it would have done to any
human habitations. Every wandering animal
and any chance hunter or traveler who hap-
pened to be near was, of course, instantly
killed. But the catastrophy was almost in-
stantaneous; in a few seconds all was over,
and ere long even the air was still once more.
The silence of death and desolation remained
there for centuries thereafter, because the
Indians held the spot to be holy and they
dared not approach it. For had not a god
descended from heaven to take up his abode
in the crater?
But as years went on white men arrived,
who had no fear of sacred places. They saw
the great crater and wondered at it. And it
was noticed that many strange masses of iron
lay scattered for some miles around. Some of
these eventually reached the hands of scien-
tists who identified them as meteorites. As
meteorites have a commercial value many
hundreds were then collected and sold far and
wide. So that the "Canon Diablo" meteor-
COLLISIONS OF COMETS AND EARTH 195
ites as they are called are probably the most
widely distributed of any.
The great excavation of which we have been
speaking is now generally known as "Meteor
Crater." Briefly it is a hole with very steep
or almost precipitous sides, nearly round, and
4200 feet in diameter. The rim, made up of
debris of all sizes, rises about 125 feet above
the plain. Inside the depth is about 500
feet, but this does not measure the depth of
the hole, for the bottom is filled by broken
masses and rock-flour to an additional depth
of 650 feet.
The immediate neighborhood is non-vol-
canic and the rocks, which consist of lime-
stones and sandstones only, lie in horizontal
strata. Around the crater, on the outside,
many meteoric masses were found, some as
much as 4 or 5 miles away. The largest of
these weighed several hundred pounds. In
the rim were found numerous iron shale-balls,
as they were called. Some of these were also
found mingled with the debris inside. Practi-
cally no meteoric material of any size was
found mixed in the rock-flour at the bottom,
though many shafts were sunk there. The
exposed strata on the inside slopes are all
196 COMETS
horizontal except on the south. Here there
is a considerable arch, inferring something
had forced the strata upward at this point.
Once scientists had visited the crater,
theories of its origin were advanced. At
first the opinion was held that it was explosive,
i.e., volcanic in origin. This was put forward
by eminent men 1 and went almost unchal-
lenged until about twenty-five years ago.
However, in 1905, D. M. Barringer of Phil-
adelphia, a mining engineer, who was also a
scientist, after close study published 2 his con-
clusions. In these he advanced an impact
origin due to a great meteoric mass. At first
his views met with little favor, even violent
opposition. But as time went on further
work proved him correct most conclusively.
Finally in 1920 a drill-hole, right through
the top of the arch mentioned, struck meteoric
material over 1300 feet down. There is every
reason to believe the main mass lies buried
there. Mining operations are now in progress
with a view to recovering the main body.
Analysis of fragments show the usual iron-
nickel composition, but also enough platinum
1 G. K. Gilbert, Science, 31, 1896.
' Proc. Acad. Nat. Sci., Philadelphia, Iviii, 881, 1900.
COLLISIONS OP COMETS AND EARTH 197
and iridium per ton to make it one of the
richest mines in the world, if only the ore can
be reached in quantity.
The writer has for some years been in close
touch with this most interesting investigation.
It has led him to conclude that what we have
here is a collision with the head of a small
comet. The nucleus must, however, have
consisted of separate masses packed together
more closely than usually assumed. As the
hole is 4200 feet in diameter it has been con-
ceded that the cross-section of the striking
mass must have had a diameter at least one-
tenth as large. In addition for some miles
on either side, i.e., this distance out into the
coma, there were isolated masses which
formed the meteorites, picked up on the sur-
face around the crater. As for the shape and
mass of the main body, indirect investigations
by various scientists who have just attempted
to solve the problem lead to widely differing
results. By analogy with comets in general,
as well as other reasons, the writer has been
inclined to accept an approximately spherical
shape, with a diameter not less than 400 nor
more than 800 feet.
Frankness forces the confession, however,
198 COMETS
that so many unknown quantities are present
that any solution must depend on assumptions
in part, and the truth will only be known when
the miners actually penetrate the mass with a
tunnel. We do not even know but that only
one great mass will be found, though we believe
there is every reason to think it consists of
many smaller ones packed together. In any
case when the mining is brought to a success-
ful conclusion, we will at last know what the
nucleus of a comet looks like, and how it is
constituted. This knowledge will be of in-
calculable value, as it will help prove or dis-
prove many theories presented in various works
on astronomy and geology. Its bearing on
theories of evolution will be especially
important.
THE SIBERIAN METEORIC FALL
On the early morning of June 30, 1908, took
place the most remarkable astronomical event
of the twentieth century, namely, the fall of
an immense object from outer space upon the
Earth. It struck in central Siberia, 8 400
miles north of the Transsiberian railroad, and
' Scientific Am., 139, 42-4, 1928.
COLLISIONS OP COMETS AND EARTH 199
in a region sparsely inhabited by a few native
tribes. Due to these circumstances, and to
the fact that there were no scientists near the
point of fall, the circumstances were not gener-
ally known for many years afterwards, and
it was not indeed until 1927 that an expedition
under Prof. L. A. Kulik finally penetrated to
the spot itself.
It is not possible here to enter into the de-
tails of how the story was gradually pieced
together, until at last we have a complete
account of the event itself, as well as descrip-
tions of the region as it now is. But the
known facts will be given those from scientific
sources first. The concussion due to the
blow against the Earth's surface was recorded
on a seismograph at Irkutsk, 900 km. away
and the air-wave at the same city. The latter
was also recorded by a barograph at Kirensk,
over 400 km. away. Sounds were heard to
the south and east of the end point for a dis-
tance of more than 1000 km. About the same
date a meteorite fell in the Kiev government.
The approximate position of place of fall
is longitude 101 east, and latitude 60 north.
It lies in a region in which mountains, rivers,
swamps and forests abound. The particular
200 COMETS
spot is a plateau between two rivers, the Pod-
kamennaja Tungaska and the Chunia, and is
quite surrounded by mountains, making a sort
of natural amphitheatre.
Although it was nineteen years after the
event before the party of Professor Kulik
penetrated to this place, thus giving a chance
for considerable new growth to take the place
of any that had been destroyed, and for ero-
sion to partly obliterate the effects, still what
he found was most amazing. In the central
area, having a diameter of perhaps 2 miles,
there was a sort of shallow depression, the
ground showing signs of having been violently
pushed sideways, as for instance when a stone
is dropped into thick mud, so that ridges still
could be seen. Inside this area were 200
"shell-holes' ' or craters, varying from those
only a yard or two in diameter to one 50
yards across. These had steep sides but due
to the swampy nature of the ground and the
artificial depression they were mostly filled
with mud or mire.
In this area proper the trees had been
destroyed, but for miles around on every side
could be found trees by the tens of thousands
lying with their tops away from the center,
COLLISIONS OF COMETS AND EARTH 201
where the meteorites had struck the ground.
The few trees that here and there remained
standing in somewhat protected places were
stripped of their bark and branches. Those
as well as the trees on the ground were seared,
as though touched by the heat from a giant
torch. It is stated that the total affected area
was from 30 to 40 miles in diameter.
Attempts to dig up a meteorite from the
bottom of one of the small craters were un-
successful, due to the swampy ooze and water
that constantly filled the depression. It is
quite impossible to make an intelligent esti-
mate as to just how deep these masses lie.
But in 1928 a few meteoric fragments were
found on the surface. Kulik is said to have
estimated the total weight of all the fragments
as being about 40,000 tons. Certainly those
that made the largest craters must have been
yards in diameter or they could not have dug
out so large a hole.
Accounts by native Tungus, who were rela-
tively near the spot, and by Russians at a
great distance, who yet heard the sounds or
felt the effects, agree in the main details.
The day was clear, and the terrible noise was
likened to thunder or artillery fire. The
202 COMETS
appearance itself to an observer at Kansk,
600 km. distant, was described as a circular
glow half the size of the Moon, of a bluish
tinge, and moving rapidly. The glow left a
bluish track, stretching along almost the whole
path and afterwards gradually disappearing
from the end.
A vivid description was given by a Russian,
steering a large boat on the Angara River.
His account was as follows: "In the north a
bluish light flashed, and from the south a
fiery body, much larger than the Sun, flashed,
leaving a broad light band. Then such a
cannonade arose that all the workers ran into
the cabin forgetting the danger from the
rapids. The first sounds were weaker but
quickly grew." The sound effect, on his
supposition, lasted about three to five min-
utes. The force of the sounds was such that
the boatmen were completely demoralized.
One native Tungu had a herd of 1500 deer
feeding in the region; of these most were lost,
only the carcasses of some being found.
Another native, "three days' march " from
the place, had his hut knocked down, the top
blown away by the wind, his brother stunned
and his deer scattered. Another man at
COLLISIONS OF COMETS AND EARTH 203
Varovara at a distance of over 30 km. de-
scribed it thus: ". . . . in a northwest direction
appeared a kind of fire which produced such a
heat that I could not stand it And this
overheated miracle I guess had the size of at
least a mile. But the fire did not last long, I
had only time to lift my eyes and it disap-
peared. Then it became dark, and then
followed an explosion which threw me down
from the porch about six feet .... but I
heard a sound as if all houses would tremble
and move away. Many windows were
broken, a large strip of ground torn away, and
at the warehouse the iron bolt broken/'
On this date the Earth was very close to
the orbit of Pons-Winnecke's Comet, distant
about 0.03 astronomical unit only. This
comet in 1916 gave a fine little meteoric
shower, and since then we have had many
splendid fireballs at this epoch, some of which
doubtless follow the same orbit. There is
some real reason 4 to suppose that the masses
falling in Siberia moved in this same orbit
and were a fragment of the original comet,
broken off ages before, and itself forming a
small comet.
4 V. A. Maltzev.
204 COMETS
Everything apparently cau be explained on
this supposition, namely, that a small comet
entered our atmosphere on the date men-
tioned and having traveled through it for some
hundreds of miles from S.S.W. to N.N.E. on a
sloping path, finally struck the ground. The
pattern of the craters is exactly what would
be expected from a nucleus of such small total
mass, whose separate units would be not very
close together. The accompanying coma, if
indeed such a small nucleus could possibly hold
one, was simply lost or mingled in the air
that, once the nucleus had penetrated our
atmosphere, was forced between the solid
units. As these came down they carried in
front of them a great mass of air, under
terribly great pressure which increased as they
came lower, and which was also at high
temperature. This immense piston of super-
heated air was what did the major damage;
when it struck it annihilated things in the
central area, then escaping sideways it tore
down and seared the forest for miles around.
The crater or pits were, of course, made by the
solid masses themselves, but the damage done
by them was small compared to that by the
air blast.
COLLISIONS OF COMETS AND EAKTH 205
It need hardly be added that had this
occurred over a city or in a thickly populated
area the resulting loss of life and damage to
property would have been appalling. It seems
there was quite enough energy to level any
city and superheated air to kill all of its inhabi-
tants, much as St. Pierre was destroyed in
1902, though from another cause.
Attention having been brought to such
craters by these two great examples search
made elsewhere has revealed several probable
cases. One of them, 5 530 feet in diameter, 18
feet deep inside, and with a rim only 2 or 3
feet above the plain, has been found in Ector
County, Texas. Small pieces of magnetic
metal were picked up around this in a pre-
liminary survey.
Another interesting set of craters, 6 which
was very probably meteoric or comctary in
origin, is found in Esthonia on the island of
Oesel. The main pit is 300 feet in diameter,
surrounded by a wall or rim which stands 15
to 20 feet high on the outside. Inside is a
small lake. Around it lie a dozen other holes
8 Proc. Acad. Nat. Sci., Philadelphia, Ixxx, 307, 1928,
D. M. Barringer, Jr.
Scientific Am., 139, 45, 1928.
206 COMETS
ranging from 15 to 100 feet in diameter. The
rocks on the main crater's rim are tilted up,
and below the floor deposits of dolomite
powder and larger stones are found. These
facts, as well as the crater's shape, make a
meteoric origin likely, as they parallel what
are found at Meteor Crater, Arizona. It is
hoped that exploration of some of the smaller
craters will be undertaken, as it appears that
meteoric fragments should be found relatively
near the surface, if this theory of their origin
is the correct one.
CHAPTER XV
ORIGINS OF COMETS
To explain how comets originate is such a
difficult problem that many writers of books on
general astronomy make no serious attempt
even to suggest a theory. Others dismiss the
question with as few words as possible, leaving
the reader quite bewildered as to what the
writer intended to advance as a tenable opinion.
Further, it is difficult to find any place for
comets in the Nebular Hypothesis of Laplace,
which was so generally accepted during the
nineteenth century.
First, Carrington may be quoted, 1 who
showed that if comets moved according to
chance distribution through space, then more
would meet the Solar System than could over-
take it. Also if they originated outside of our
system, orbits with eccentricities much over
unity, i.e., strongly hyperbolic, would be the
rule rather than the exception. We find that
comets' orbits fulfil neither condition.
Miss Agnes Clerke wrote as follows, 2 "We
Mem. R. A. S., 29, 335, 1860.
1 History of Astr., 370, 1902.
207
208 COMETS
conclude then that the 'cosmical current/
which bears the solar system towards its un-
known goal, carries also with it nebulous
masses of undefined extent, and at an unde-
fined remoteness, fragments detached from
which, continually entering the sphere of the
Sun's attraction, flit across our skies under
the form of comets. These are, however,
almost certainly so far strangers to our system
that they had no part in the long processes of
development by which its present condition
was attained. They are perhaps survivors of
an earlier state of things, when the chaos from
which the Sun and planets were to emerge had
not yet separately begun to be."
The two possibilities, as seen twenty years
ago, is thus summed up by Chambers: 3 "Two
provisional answers suggest themselves:
either (1) comets are chance visitors wander-
ing through space and now and again caught
up by the Sun, or by some of the major planets
.... and compelled to attach themselves to
the Sun and by taking elliptic orbits to be-
come permanent members of the solar sys-
tem; or (2) they are aggregations of primaeval
The Story of the Comets, 184, 1910.
ORIGINS OF COMETS 209
matter not formed by the Creator into sub-
stantial planets, but left lying around in
space to be picked up and gathered into en-
tities as circumstances permit."
In the 1922 edition of Newcomb-Engel-
mann, page 439, we find it stated that "the
most probable opinion as to the origin of
comets is that they take their origin in collec-
tions of matter, which at great distances
accompanies the Sun in its wandering through
space."
Leuschner 4 expresses his opinion as to the
origin of comets as follows: "The short period
comets, according to the well-known 'capture'
theories are supposed to have had their orbits
changed by Jupiter into very short-period
planetary orbits. Such cases can actually be
traced. It is natural to suppose that comets
in general represent the left-over material
from the original nebula which did not con-
dense into the Sun or one of the major planets.
Not all this material may have been left over
at the outskirts of the solar system beyond the
orbit of Neptune. We are justified in assum-
ing that there may be a belt of such left-over
<Pub. A. S. P., 39,294, 1927.
210 COMETS
material at least about the largest of the major
planets which is Jupiter and that the Jupiter
group of comets has come into existence from
Jupiter. This of course does not preclude the
capture of some of the comets coming from
the outskirts by Jupiter and the change of
their orbits into short period orbits. Whether
originally near Jupiter or at the outskirts of
the solar system comets in general may repre-
sent the original condition of minor planets."
The ideas above expressed fall in general
under the theory known as "The Home of the
Comet." This "home" or nebulous shell
must partake of the motion of the Sun through
space towards the solar apex. Its distance
may be estimated at from 10 4 to 10 B astro-
nomical units. Bosler 5 says: "Strange as this
idea may appear, it is only a literal translation
of material facts: it is not even a hypothesis,
but a convenient manner of depicting objec-
tive reality, as it has been revealed by modern
research."
However, this theory gives us no inkling as
to how such a body as a comet could form
under the conditions that must be expected to
5 Astrophysique, 438, 1928.
ORIGINS OF COMETS 211
exist in such a nebulous shell, or why a given
mass of it should start towards the Sun, while
others did not simultaneously do so.
Crommelin, one of the greatest living
authorities on comets, seems somewhat in-
clined to consider a solar origin possible for
comets which have their perihelia near the
Sun, especially for the group including the
Great Comet of 1882. He bases this upon
the knowledge that solar prominences are
driven off with immense velocities. He feels
that this explanation is not adequate for
comets whose perihelion distances are unity
or greater.
R. A. Proctor about sixty years ago sug-
gested that the comets were expelled from the
planets to whose families they "belonged."
At present, recent researches have proved
the outer visible surfaces, which are, however,
merely the tops of cloud layers surrounding
Jupiter and Saturn, are of the order of 140
and - 150 Centigrade. But Crommelin, 6 in
view of the appearances of spots on the plan-
ets, which were evidently deep-seated in
origin, thinks it possible that matter may be
6 Ency. Brit., xiv, 5, 103, 1929.
212 COMETS
driven away from these bodies at such speed
that it will not return. Something analogous
to super-volcanic eruptions is meant. If such
matter had velocity enough it would leave the
planet and form comets. If this theory is
true, an explanation would be given as to how
a fresh supply of comets is produced, to take
up the wastage which we see constantly going
on.
Schiaparelli 7 thought comets formed a sort
of stellar current, moving along with the Sun.
He said: "The meteorites may be comets of
other suns, which, under their heating action
have already by frequent and great emissions
of jets and tails lost all or nearly all of their
containing gases; while our Sun has not yet
extracted from all its comets and dispersed in
space the total amount of the gas that they
originally contained. Finally, comets and
meteors may differ among themselves only in
the diversity of the places attained in their
evolution."
Baldet 8 basing his conclusions on the studies
made at the 1927 return of Pons-Winnecke's
Comet, as well as on extensive work done by
Bui. Astr., 27, 250, 1910.
8 Bui. Soc. Astr. de France, 41, 401, 1927.
ORIGINS OF COMETS 213
him on others, says: "Even supposing that
Pons-Winnecke's Comet came from infinity
and was captured by Jupiter, which is not
demonstrated, we must admit that it is of
relatively recent formation. The same rea-
soning applies evidently to other comets
But it is sufficiently remarkable to note, in
finishing this study, that modern work upon
comets, both in astronomy of position and
astrophysics, lead us little by little to see them
as of recent formation; it is even possible to
think that comets must have their birth in
our own time."
Baldet further quotes Crommelin's opinions
as to planetary origins with at least tentative
approbation.
Chamberlin in his The Tjpo /Solar Families 9
has advanced another tiypothesisT which has
certainly the recommendation that it is com-
plete in itself, whether it can be adopted or
not. In fact it is so fully explained that it is
difficult to give a brief yet fair r6sum6.
The validity of Chamberlin's conclusions
depend, almost wholly, upon the correctness
of his assumptions about the formation and
9 Page 251 et sec.
214 COMETS
actions of his smallest unit, called a chondru-
lite. He calls attention to particularly vigor-
ous eruptive prominences on the Sun, and to
the fact that in some cases the parts received
additional impulses after leaving the Sun's
surface. This material 10 is first hot and
gaseous, "but by reason of its divergent pro-
jection, its intrinsic expansion, and its radia-
tion, as it sweeps out into interplanetary
space, it is rapidly cooled below the volatile
temperatures of the main materials that make
up the chondrulites. These are thereby
forced to form precipitates, and these in time
naturally aggregate as they are forced one
against another by the agitation in the pro-
jected mass. The minute accretions are the
primitive chondrulites. They embraced
practically all kinds of matter precipitated."
Also they were subject to sharp collisions.
This would explain their fragmented struc-
ture. "The scattered and mixed state of the
fragments is clear evidence that the chondru-
lites were not formed in place."
In the early stages of the driving out of
these hot gases, the conditions were certainly
10 LOG. eft., p. 262.
ORIGINS OP COMETS 215
dispersive. But when such matter reached
zones beyond the last planet, such action was
partially screened by intervening matter,
while for the same reason the gravitational
pull towards the Sun was increased. So the
chondrulites in general lost speed the further
out they went. Most of them would eventu-
ally be turned back, and would then have to
revolve about the Sun in elliptic orbits of very
high eccentricity. Some of them also had
grown constantly by accretions, becoming the
true chondrulites. The latter were thus a
product of selection; also when any one
reached a given size (i.e. mass) it inevitably
turned back towards the Sun, gravitation
overpowering the forces of outward propulsion.
Also in this outer zone where their velocity
was almost zero, their mutual attractions
became more of a factor. So when such a
mass of chondrulites were moving in nearly
parallel paths, Chamberlin believed that in
this outer zone their mutual attractions would
tend to make them gather into swarms or
groups. Such swarms became the heads of
comets; those chondrulites which did not so
gather became the ordinary sporadic shooting
stars or meteors. Further the idea is ex-
216 COMETS
pressed that comets, on subsequent visits to
this outer zone after a perihelion passage,
partly replenish their stores of fine dust and
gas, which are considered as being present in
some quantity in this zone.
The detailed behavior of comets is dealt
with at length by Chamberlin, after he has
explained the above hypothesis of origin.
But as pointed out the whole stands or falls
on the assumption that chondrulites are
formed in the manner outlined.
Recent observations by Evershed 11 bear
most favorably upon the above theory. "It
is curious to observe on these photographs the
intricate structure assumed by a mass of gas
projected into space and freed from the re-
straining forces in the photosphere. It does
not diffuse away uniformly, but appears as a
mass of fine filaments and interlacing streams
of gas, seemingly knotted together at one
point, which suggests that light pressure
alone is not an adequate explanation of these
eruptions." Reference is here made by Ever-
shed to a rare type of prominence seen on
November 19, 1928. This at 7 h 50- I.S.T.
" Observatory, 52, 38, 1929.
ORIGINS OF COMETS 217
was 3' to 4' high, at 9 h 3 m was 21' high ; still re-
maining bright when clouds came over.
In a recent paper by N. T. Bobrovnikoff 12
the question of disintegration of comets is
studied, and his investigations bring him to
interesting conclusions about their origins.
First he proves, basing his results on 94 peri-
odic comets, that there is a definite correla-
tion between the absolute magnitude of a
comet and the value of the dispersive function
4 (a, e) derived theoretically. And also that
the abundance of gases in the head and the
tail of comets shows correlation with the
same function < (a, e). In other words that
both show certain dependencies on the major
axes and eccentricities of the orbits.
These conclusions are partly based on
work by Holetschek who proved that the
maximum length of comets' tails depended
upon their absolute brightness, and by Vsech-
sviatsky who showed that the average abso-
lute brightness of comets increased with the
increase of the inclinations of their orbits.
Bobrovnikoff says: "In their obedience
to the law of disintegration comets form a
" Lick Obs. Bui., No. 408, 1929.
218 COMETS
system all members of which follow the same
career with the same ultimate fate. The
matter in comets must be in a very peculiar
state of excitation which does not occur often
in the universe. Yet all comets have func
jnentally the same IspeHrum. Tliesefacts
constitnfe T strong evidence ior the common
leTsystenTof comets is closed, that is, we
have no influx of fresh comets coming from
the depths of the universe. If we had any,
it would affect the correlation between their
orbital elements and the average absolute
brightness. Indeed such correlation would be
incomprehensible.
"On the other hand, this correlation shows.
that the disintegration of comets is an irrevers-
ible process. There is no building up of new
comets from the meteoric and gaseous matter
diffused in space ..... "
He then develops the argument that comets
can scarcely be more than 1,000,000 years old,
for if older they must before this have wholly
disintegrated. He also shows that comets
are evidently true members of our Solar
System, yet that their ages are not com-
parable to that of the planets. Other investi-
ORIGINS OF COMETS 219
gators have shown that the distribution of the
perihelia of comets' orbits show regularities
which can scarcely be explained, in his opin-
ion, if these bodies have been in the Solar
System from its origin. His solution is that
all comets were captured, at about the same
time, and not longer than a million years ago.
The Sun appropriated them during one of its
passages through diffuse clouds of obscuring
matter.
In comment on this theory it may be said
that in common with nearly all others, while
it gives a possible place and time from which
comets came, it offers no explanation as to
why bodies of such peculiar structure were to
be found there, or as to how matter in such a
diffuse cloud could form into comets. The
theories which have been outlined are those
which have been advanced during the last
half century and which, in the writer's
opinion, deserve consideration.
CHAPTER XVI
CONCLUSIONS
"Wandering stars, to whom is reserved the black-
ness of darkness forever. 1 '
In such a difficult problem as that of ex-
plaining the origin of comets, and one which
many eminent men have attempted with in-
different success, it would be presumptuous to
believe that an entirely satisfactory solution
can be given in the present state of astro-
nomical knowledge. In the previous chapter
many opinions have been quoted, and certain
difficulties pointed out. Here the writer will
discuss the subject from his own viewpoint,
hastening to add that he does not claim to
have reached a satisfactory solution, but can
only hope to indicate what may prove to be
useful lines of approach.
First, the origin of comets can scarcely be
discussed without some previous knowledge
about that of the evolution of the Solar Sys-
tem in general, that is, unless we accept
BobrovnikofFs theory that comets have re-
cently been acquired by the Sun, and are not
really members of his family, but only adopted
220
CONCLUSIONS 221
children. Or we might adopt the theory that
they are still being created by planetary
eruptions. This former theory has attractive
features, the best being that it disposes of the
difficulty of explaining how comets could have
lasted as long as the planets have, i.e., were
created at the same time as these latter. Ad-
mitting this excellent feature, still it does not
appear wise to deny a solar origin to comets
until all possible (and reasonable) explana-
tions have been exhausted. As the writer does
not believe that this has yet been done, cer-
tain ideas will be outlined which may offer a
possible explanation of the time difficulty in
an assumed solar origin. It is further useful
to do this, for in Bobrovnikoff 's theory, which
assumes capture of the comets as the Sun
passed through a nebulous region, no mecha-
nism is suggested as to how such a body could
be formed under the conditions that are sup-
posed to exist in a nebula. KJT \ ffill'g <3**s*n
In all that follows a catastrophic origin of
the planetary system, due to the near passage
of another star to our Sun, is postulated. It
seems quite certain that the difficulties raised
against the Nebular Hypothesis of Laplace
are insuperable, so this need not be discussed
222 COMETS
here. The Planetesimal Hypothesis 1 of Cham-
berlin and Moulton is, in its broad out-
lines, assumed as explaining correctly the
genesis of the planets of the system. The
various modifications of this hypothesis, due
to certain English astronomers, 2 while differ-
ing in details, are still in the writer's opinion
based wholly upon the one mentioned as to
fundamentals, and would not have been ad-
vanced had not the other first appeared.
The latest exposition of the Planetesimal
Hypothesis 3 was written by Chamberlin, just
before his death. Here he gave his personal
ideas as to the formation of comets and me-
teors, which formed the second of the solar
families discussed. The writer, while accept-
ing in general the first part of the book,
namely, that giving the original theory of the
formation of the planets, has to disagree with
the second half. But he believes that the
ideas expressed in the first half may be used
in a more correct explanation of the origin of
comets and meteors.
1 F. R. Moulton, Introduction to Astronomy, Chap, xv, 1906.
1 Sir James Jeans, Problems of Cosmogomy and Stellar
Dynamics, 1919.
8 Chamberlin, The Two Solar Families, 1928.
CONCLUSIONS
223
The planets are assumed to have come from
the zones near to and parallel with the Sun's
equator. Here the tidal forces were great
and were aided by the explosive forces lying
in the spot zones. It seems, however, that in
addition polar eruptions would be caused,
though of smaller amount, for the following
reason.
In figure 4, (a) represents the undisturbed
spherical Sun, (6) the Sun with the immense
tides, which are due to and pointing towards
and from the disturbing star Y. X* is of
course exaggerated as to probable heights of
the tides. Now the material that heaped up
at Q and E had to come from somewhere, and
left a "low-tide" belt at right angles, that is
extending completely around the Sun, half
way between Q and E, and passing through
224 COMETS
each pole. This meant that, surface material
being removed, hotter and presumably more
active or explosive material was brought to
or near the surface. Would not great erup-
tions of this lower-lying material then take
place all around this belt, including the part
of it passing through the polar regions, which
are usually relatively quiescent? Consider a
mass M thrown out from N' and one R from
S'. If projected vertically, the attractions of
the other star would draw them over toward
it, in other words give curvature to their
paths. But the directions of their motion
would be exactly opposite. Here we have an
easy explanation of masses in our system
which from birth had retrograde motions, as,
off-hand, we may say that at least half of these
masses went one way, half the other. If this
explanation can be accepted, we get all possible
inclinations for such bodies. Also, if their
material was in general lower-lying than that
which was torn off at Q and E to form the
planets, bodies, differing somewhat from the
latter in their constitutions, might be expected.
Another point is that material torn from the
other sun would mingle with that torn from
our own, and so would be available in form-
CONCLUSIONS 225
ing bodies in our system. Both motions and
physical' conditions might be largely modified
thereby.
The idea here advanced is that comets had
such an origin, also many ngtipmidfl an d per-
haps some bodies that are now satellites of
certain planets. The argument in favor of the
inclusion of these two latter classes will now
be developed.
That there can be no hard and fast line of
demarcation between asteroids and comets is
proved, for instance, by asteroids Nos. 944
and 1927 BD. 4 Their orbits are quite comet-
ary. If these bodies showed either a coma
or a tail they would be classed as comets.
Both objects are, however, sharply stellar in
appearance. Other asteroids have orbits of
high inclination and large eccentricity, not un-
like short period comets. It will be objected
that we have no asteroids with retrograde
motion, while half of the comets have it.
But let us point out that all very short period
comets have direct motion and small inclina-
tions. There are a few comets of medium
period and retrograde motion. Cannot this
4 Meteors, 264 et aeq. Also Jour, of Franklin Inst., 207, 733
1929.
226 COMETS
be interpreted to mean that comets (or aster-
oids) of short period, retrograde motion, and
small inclinations had a poor chance for sur-
vival? They were, to express it crudely,
swimming against the stream, when, accord-
ing to hypothesis, the whole region within the
orbit of Jupiter must have been filled with
planetary building material. Therefore, in
time, collision with direct moving material
could scarcely be avoided. The possibility is
then suggested that asteroids and comets had
the same kind of origin. In the first case it
seems probable that enough material came
together under favorable circumstances for its
mutual gravitation to draw it all into a com-
pact or comparatively compact body. In the
second case, the circumstances were unfavor-
able and the material less.
With regard to both asteroids and satellites,
it seems to have been generally tacitly as-
sumed tjiat they were solid, continuous
bodies. But on what is this based? So far
as asteroids are concerned we have no way to
determine the masses of any of them, nor are
we liable to do so unless in the cases of the
very largest a comet should pass extremely near
one of them. With satellites we are in a more
CONCLUSIONS 227
favorable position, and very curious results
follow in some cases. For instance the fourth
satellite of Jupiter Callisto turns out to have
a density of only 0.58. Also for the satellites
of Saturn we find Mimas, 0.24; Enceladus,
0.52; and Tethys, 0.54. These values are cal-
culated from data in Table IV of Astronomy
by Russell, Dugan and Stewart. How can
such low densities be explained on the basis of
a single, continuous, solid body? For it
seems impossible to assume these small bodies
are still in the gaseous or liquid form. Is it
possible that such satellites furnish us with
examples of bodies that are in constitution
half way between a typical solid satellite or
asteroid, and the nucleus of a typical comet?
Is it possible that the variability of some as-
teroids and satellites could be explained on
the basis of their being a rather compact
group of separate masses rather than one
continuous body? Remember thatpracti-
cally all asteroidsWe can see are^within the
orbit of Jupiter ; we do not know about those
OTrtsidSI Again we find meteorites and fire-
balls with orbits of all inclinations. Does it
not seem probable, therefore, that the action
of Jupiter, plus collisions with other masses,
228 COMETS
did not permit retrograde asteroids of con-
siderable size to survive? Very small ones
with orbits of high inclination would escape
discovery except under most favorable cir-
cumstances. But these actions just mentioned
might well permit the survival of retrograde
comets of long period, which would spend so
little of their life within the critical region.
Calling up again the Meteor Crater and
Siberian cases, here we have one body, which
if it could have been seen at all would have
looked like a tiny asteroid, the other like a
tiny comet. Yet each are technically comets.
Again recent work at the Lick Observatory
shows that spectrograms of asteroids are re-
markably weak in the ultra-violet region and
this, in the opinion of the observers, connects
these bodies physically with the nuclei of
comets. Also the presence of small retro-
grade satellites in the system of Jupiter and
Saturn, outside the orbits of the direct moving
satellites of these planets, proves that retro-
grade bodies of small size survive only under
special conditions. So while the chain of evi-
dence is far from complete, these various
facts are certainly very significant. The
probable reason we see so few cases of
CONCLUSIONS 229
"stripped nuclei" of comets to borrow a
physical term such as the Meteor Crater
nucleus possibly was, is that such a comet,
stripped of its coma, assuming indeed it ever
had one, would usually be too small to be seen
at any distance from the Earth. We can by
no means assume that many do not pass us
annually undetected.
Next as to the time element, which is a
most serious difficulty. Let us assume the
time interval since the formation of our sys-
tem to be 8,000,000,000 years, a figure now in
good repute. Then a comet with a period of
1000 years would have returned 8,000,000
times, and so on for any period we may choose.
With this in view, we must, acknowledging
the facts of cometary disintegration, admit
at once that all comets which originally had
short periods have long since disappeared as
comets. They may survive as asteroids, in
given cases. The same fate doubtless befell
those with original small perihelion distances.
To explain the present state of things we are
then forced to postulate that all comets of
short or even moderate periods have been
recently turned into smaller orbits, through
planetary influences, or by passing through
230 COMETS
other resisting media found further out in
space. This is simply an extension of the
capture theory on a much grander scale.
In explanation, the idea is that it would
not make any difference how often a comet
came to perihelion, so far as its being disrupted
or losing its gases are concerned, provided only
its perihelion distance was great. We are
not held to any particular figure for the prob-
able critical distance, but it may be that a
nearest approach of 10 astronomical units
would still leave a comet unaffected. If this
limit is considered too low, fault could hardly
be found with 20 or 30 units, as the limiting
distance. This would still leave the comets
subject to planetary perturbations, which in
the course of ages would switch a certain num-
ber into smaller orbits.
It may be added that we have no data what-
ever on comets with perihelia as much as 5
units from the Sun, so disproof of this hy-
pothesis is at present difficult. That it postu-
lates the evolution of an immense number of
comets, when our system came into being, is
obvious. That further it requires the "cap-
ture" of a certain number almost yearly in the
sense that their orbits are being made gradu-
CONCLUSIONS 231
ally smaller so that in time they come into
our range of vision, is also true. Mathemati-
cal investigations of such a problem in iso-
lated instances have been worked out by
many, so the process is more or less
understood.
With full realization that nothing at present
further than probability can be adduced in
favor of this theory, it is still advanced in the
hope that it may stimulate others toward
attempting its proof or disproof.
From our studies it is believed that we
have a fair idea of what composes a comet,
and how the matter is arranged. Then the
last question comes up, how in detail did
such a curious and complex body evolve?
Would the same forces that formed the planets
suffice for comets? Here the writer admits
that he has not even an intelligent guess to
offer in addition to what has already been
said, and this he believes is wholly inadequate.
He only ventures to say, as a mere matter of
opinion, that in the chaotic and diversified
conditions following the birth of our planet-
ary system, it would appear that there would
have been more chances for the evolution of
such bodies than in a nebula.
232 COMETS
In conclusion, comets still offer some of the
most surprising phenomena and perplexing
problems in the whole realm of astronomy.
It is not improbable, indeed, that the mystery
of their origin and formation will be the last
problem, dealing with the evolution of the
Solar System, which astronomers of the future
will be called on to solve.
APPENDIX
For the benefit of those readers who are not familiar
with the elementary facts about orbits and yet desire a
little information on the subject, the following pages
have been added.
First it should be said that comets, as we have seen,
move in three possible types of orbits about the Sun:
the ellipse, the parabola, and the hyperbola. In all
cases the Sun occupies a focus. The Earth itelf
revolves around the Sun once per year in an ellipse
that differs but little from a true circle, in other words
has a small eccentricity actually equal to 0.0167.
The semi-major axis of our orbit, technically called the
mean distance to the Sun, is known as the astronomical
unit, since it is the unit used for all larger distances
within the Solar System. It equals 92,870,000 miles or
149,450,000 kilometers. The plane of the Earth's
orbit is known as the plane of the ecliptic. This is
used as the plane in the Solar System to which we refer
other orbit planes. The circle in which this plane
cuts the celestial sphere is the ecliptic itself, the path
followed by the Sun in its apparent annual circuit of the
heavens as well as that followed by the Earth as seen
from the Sun.
In order to define the size, shape and position in
space of the orbits of other bodies, as well as their
positions in their orbits, so-called elements are de-
rived. For a body moving in an ellipse, seven are
necessary; for one in a parabola, only five. This at
233
234
APPENDIX
once indicates why the latter type of orbit is easier to
compute. In the ellipse, two (a, e) define its size and
shape; three (i, ft, ) define its position in space; two
(Pe, T) define the position of the body in its orbit at a
given date. Using figure 5, we define these quantities
as follows:
(1) a
(2) e
(3) f
Fio,5
OA = OP The semi-major axis of the ellipse,
defining its length.
OS OS
sin <f> The eccentricity of the
ellipse, obviously a ratio defining its semi-
minor axis OB = 6 = aVl c 2 .
/.CJfEt The inclination of the plane of the
orbit to the plane of the ecliptic. If i < 90
the motion of the comet is direct. If t > 90
the motion is retrograde.
APPENDIX 235
(4) 12 = /.A'SN The longitude of the ascending
node, which is the angle measured in the plane
of the ecliptic between the lines SA' and SN.
Where the line SN cuts the orbit is the point
in which the object comes from below the
plane of the ecliptic as it moves towards A',
i.e., the First of Aries.
(5) w = Z.NSP The angle, measured in the direction
of the comet's motion, between the lines SN
and SP.
(6) Pe The period, i.e., the time interval in years and
fractions that it takes the body to make a com-
plete revolution around the Sun.
(7) T The epoch, the date at which it passes P.
We should further define P as the perihelion point,
i.e., that point on the orbit nearest to the Sun. SP =
q = a (1 e). Also A, the aphelion point, is the most
distant point on the orbit from the Sun: SA =
a(l + e) . In place of (5) many books give the longitude
of perihelion TT = 12 + w. Obviously if 12 and w are
known, TT is also known. It should, however, be care-
fully noted that 12 + w is the sum of two angles lying
in different planes. Also in place of (6) we frequently
have given the mean daily motion p, = 1296000" divided
by the period expressed in days. Again it is obvious
that if either /z or Pe is given the other can at once be
derived.
In the parabola, by definition e = 1, the curve is an
open one, and the semi-ma j or axis is infinite . Therefore
there can be no period. The elements i, 12, w, and T
are defined as in the ellipse. We have, however, the
236
APPENDIX
additional element q = the perihelion distance, or the
distance SP. It is needless for our purposes here to
describe the hyperbola further than to say that it too
is an open curve so that a body moving around the Sun
in such an orbit would never return.
As noted in earlier chapters most comets are said to
move in parabolas, clearly keeping in mind what the
FIG. 6
statement really means. Even most of those known to
move in ellipses have orbits of high eccentricity. This
being true, we will not make any great error if we
assume that any given comet moves, when near the
Sun, in a parabola, so long as we are dealing with a
rough graphical representation of its motion. In
figure 6 we desire to illustrate how the relative positions
of a comet and the Earth, when the former is in the
visible part of its orbit, may be approximately deter-
APPENDIX 237
mined. The figure is of more obvious value when the
inclination is small. The general method is due to
Comstock. 1
Let EiE2E 3 represent the Earth's orbit, and, as a
first approximation, let the plane of the comet's orbit
lie in the plane of the ecliptic, i.e., i = 0. Let the
line of nodes be N'SN. By known properties of the
parabola the chord through S perpendicular to SP = q
is 4 q in length .'. SXi = 8X2 = 2q. Again from
celestial mechanics we have for motion in a parabola
(1)
in which V is the angle at S between the radius vector
to the comet and SP, t is the date, and T the date of
perihelion passage, i.e., the epoch. Also k is the
Gaussian constant of gravitation, which in terms of
astronomical units per day = 0.0172.
In the figure V = 90, /. J = 45, and tan 45 =
i
V I V
1, /. tan -s + o * an8 TT == V3. Also if q = 1, we can
2 3 2
\/2
write the above equation 4 -^- = k (t T), and sub-
o
stituting the value for k, we find t T days. This
gives the time it takes the comet to travel from Xi to
P, or P to X 2 . The equation can be solved most simply
for any other value of q by the use of logarithms.
i Pop. Aatr., 6, 465, 1808.
238 APPENDIX
If SA represents the direction to the First of Aries,
and the Earth's orbit be taken as circular, then its
radius equals 1. Also the Earth will occupy E* at the
fall equinox and EI at the spring equinox. As it moves
360/365| of a degree per day, it is easy to find its
position on the orbit at any subsequent date, measuring
from one or the other of these the nearest of course.
The comet must be at P on the date of perihelion pas-
sage . We also saw above how to calculate when it would
reach Xz or was at XL Its position at any intermediate
date can be accurately calculated from equation (1)
if Barker's tables are at hand. These tables are found
in Watson's Theoretical Astronomy, as well as in other
publications. An approximation close enough for
rough purposes can be made by interpolating along
PXz, allowing for the fact that the velocity of the comet
is considerably greater at P than at X 2 .
Having thus found positions for Earth and comet on
the same dates, a line joining any two such positions
will give both the distance of the comet from the Earth
and the elongation of the comet from the Sun. This
will permit, by inspection, our finding if the comet is a
morning or evening object, and also its position on the
ecliptic. Obviously, however, the orbit plane of the
comet usually will make an angle with that of the
ecliptic. In this case consider the orbit rotated around
NSN' as an axis until the proper inclination has been
reached. If the angle is large, considerable calculation
may now be necessary, but the solutions of plane
triangles will suffice for the approximations desired.
APPENDIX 239
As a typical case let us take the elements of Comet
1927/ (Gale):
T 1927 June 14
a, 209
n 69
t 13
q 1.26
Figure 6 has been drawn to scale for these elements,
assuming i = 0. The corresponding parabola is
Xi P Xz. The second parabola, X\ P' X'%, represents
an orbit with the same elements except that q = 0.6.
As the epoch is June 14, this is evidently 8 days before
the summer solstice. Hence the Earth will have
moved 82 from EI and be at # 3 , when comet C is at P,
and comet C' at P'. We at once calculate that it will
take C 155 days to reach X 2 , but C' only 51 days to
reach X'i. As the inclination is 13, both Earth and
comet are moving with direct or counter-clockwise
motion. Hence the respective angles between the
comets and the Sun are PE^S and P'E^S. Obviously
P will rise several hours before midnight and will
remain visible until dawn. P', on the contrary, will
rise only about an hour before the Sun, and hence be
visible in the morning twilight. As i is only 13, the
error of assuming Earth and comets in the same plane
will not be a serious one in this case. Other relative
positions of the three bodies can be readily plotted and
the resulting angles read off and interpreted as above.
Also the distance of the comet from either Sun or Earth
can be measured off by a scale to quite a fair degree of
approximation.
INDEX
Airy, Sir G. B., 133.
Aitken, R. G., 113, 180, 182.
Albrecht, S., 181.
Almagest, 6.
Alpha particles, 71.
American Meteor Society, 126.
Andromedes Meteors, see
Biclid.
Anomalous Tails, 64, 75.
Apian, 10, 95.
Apollorinus Myndius, 4.
Aquarid Meteors, 125, 186.
Arago, D. J. F., 51.
Aristotle, 4, 7.
Asteroids, 18, 190, 225, 228.
Backlund, O., 54.
Baldet, F., 75, 77, 84, 88, 123,
161, 171, 173, 182, 212, 213,
220, 221.
Barnard, E. E., 25, 35, 77, 119,
120, 156, 158, 162, 165, 167,
170, 182.
Barringer, D. M., 196.
Bayeux Tapestry, 106\
Bayle, 11, 14.
Berkeland, 90.
Bernard, A., 124.
Bessel, W., 33, 65.
Bible, quotations from, 11, 16,
94, 185, 193, 220.
Biela-von, 127, 128.
Bielid Meteors, 143, 186.
Bobrovnikoff, N. T., 77, 91,
92, 116, 124, 217, 220, 221.
Bosler, J., 66, 70, 210.
Bradley, 131.
Brahe, T., 8.
Br6dikhine, T., 43, 63, 75, 76,
77.
Brooks, W. R., 35.
Burckhardt, J. K., 106.
Bouvard, A., 127.
Calixtus III, Pope, 107, 108.
Canon Diablo, 194.
Capture theory, 47, 208, 213,
219.
Carrington, 207.
Catalogues of comets, 22, 24.
Cathode rays, 65, 90.
Chaldeans, 3, 8.
Challis, J., 132, 133.
Chalons, Battle of, 105.
Chamberlin, T. C., 213, 222.
Chambers, G. F., 75, 156, 208.
Chinese, 2, 103, 104, 105, 106,
107, 153.
Chondrulite, 214.
Clairaut, A. C., 97.
Clerke, Miss A., 207.
Comets :
Coma, 16, 27, 33, 56, 86, 112,
157, 171, 179, 227.
Disintegration, 19, 41, 131,
159.
241
242
INDEX
Envelopes, Jets, 32, 60, 92,
109, 116, 150, 155, 166.
Mass, 26, 122, 123.
Nuclei, 16, 41, 85, 112, 115,
136, 151, 159, 171, 179, 227.
Origin, 207.
Tails, 2, 5, 9, 12, 16, 18, 29,
58, 86, 90, 117, 154, 159,
162, 178.
Transparency, 32, 33, 34,
157.
Comet of 1140 B.C., 2.
467 B.C., 103.
344 B.C., 2.
146 B.C., 3.
43 B.C., 3.
472 A.D., 8.
1577, 8.
1528, 7.
1607, 8.
1618, 8.
1664, 10.
1680, 10, 12.
1744, 74.
1770, LexelPs, 24, 176.
1807, 71.
1825, 74.
1843 I, 68.
1858 VI, Donati's, 61, 149,
156.
1860 III, 159.
1861 I, 186.
1861 II, Great Comet 14,
153.
1862 II, 61.
1862 III, Tuttle's, 142, 185.
1864 II, Temper s,79.
1865 f, 81.
1866 I, Tempel's, 19, 186,
187.
1867 b, 81.
1868 a, 81, 83.
1874 III, Coggia's, 81.
1881 III, Tebbutt's, 81, 82,
84.
1882 III, Great Comet, 40,
93.
1887 I, 45.
1887, Eclipse Comet, 45.
1892 I, Swift's, 78.
1892 III, Holmes's, 83, 156.
1899 I, Swift's, 159.
1903 IV, Borelly's, 74, 78.
1906 IV, Kopff's, 159.
1907 IV, Daniel's, 78, 161,
183.
1908 c, Morehouse's, 61, 68,
77, 78, 90, 123, 160, 183.
1910 a, 74, 86, 178.
1911c, Brooks's, 75.
1912 a, Gale's, 75, 76.
1913 V, 92.
1914 V, 77.
1915 e, 159.
Biela's, 19, 49, 74, 159, 186.
Daniel's see 1907 IV.
Encke's, 31, 50, 140.
Halley'8,6,11,14,28,68,72,
73,92,94,139,159,186.
Morehouse's, see 1908c.
Pons-Winnecke's, 33, 168,
186, 203, 212.
Tempel's, 19, 186, 187.
Comstock, G. C., 237.
INDEX
243
Constantinople, 107.
Copeland, R., 82.
Cowell, P. H., 102, 103, 106.
Craters-meteoric, 193, 198,
205.
Crommelin, A. C. D., 57, 102,
103, 105, 106, 211, 213.
Curtis, H. D., 68, 69, 110, 111,
114, 119, 120.
Cyanogen, see Spectra.
Damoiseau, M. C. T. de, 100,
127.
d' Arrest, H. I., 143.
Delisle, R. D., 99.
Denning, W. F., 175.
Denza, P. F., 145.
Deslandres, H., 90, 124.
Disruption, see Comets.
Di Vico, F., 130.
Donati, G. B., 79, 149.
Dorffel, 10.
Draper, H., 82.
Dumouchel, M., 109.
Earth, 25, 26, 101, 117, 129.
Eddington, A. S., 166.
Einstein, A., 45.
Elkin, W. L., 40.
Ellerman, F., 117.
Envelopes see Comets.
Encke, J. F., 101, 130.
Epigenes, 4.
Euler, L., 65.
Evershed, J., 216.
Fabry, L., 67.
Families of comets, 47.
Finlay, W. H., 40.
Fracaster, 10.
Giacobini, 35.
Gould, B. A., 40.
Gouy, 82.
Greek opinions, 2, 3, 4, 6.
Groups of comets, 37.
Halley, E., 10, 94, 95, 100, 109.
Hartwell, 141.
Helwan Observatory, 120,
121.
Herrick, 131.
Herschel, A. S., 143.
Herschel, Miss C., 50.
Herschel, Sir W., 71.
Hevelius, 109.
Hind, J. R., 102, 103, 104, 105,
106, 107.
Hirayama, K., 104.
Hoffmeister, C., 126.
Holetschek, J., 23, 217.
Holmes, 156.
Home of comets, 210.
Homer-quotation, 5.
Hufnagle, 44.
Huggins, Sir W., 80, 81, 84.
Hull, 65.
Humboldt, 3.
Humphreys, M. W., 73.
Humphreys, W. J., 117.
Huns, 105~
Jannsen, 81.
Japanese, 2, 106, 176.
Jets, see Comets.
Jerusalem, 11, 104.
244
INDEX
Josephus, 11, 1Q4.
Julius Caesar, 3.
Jupiter, 25, 47, 52, 96, 99, 101,
158, 168, 189, 209, 210, 211,
226, 227.
Kepler, 8, 65, 95.
Klinkerfues, W., 144.
Kreutz, H., 44.
Kron, E., 122.
Kulik, L. A., 199.
Lalande, J. J., 98.
Langier, 102, 105.
Laplace, Marquis de 207, 221.
Lebedew, 65, 66.
Lehmann, 101.
Leonid Meteors, 19, 148, 185,
186.
Lepaute, Madame H., 98.
Leuschner, A. O., 209.
Literature-references, 2, 5, 6,
11, 12.
Lohse, J. G., 82.
Loomis, 138.
Lubienietz, 103.
Lyrid Meteors, 186.
Maclear, Sir T., 109.
Mclntosh, R. A., 129.
Mars, 121, 190.
Maury, M. F., 132, 133, 134,
141.
Mass, see Comets.
Mechain, P. F. A., 50.
Meltzev, 175, 203.
Merrill, P. W., 179.
Messier, 34, 35, 99, 127.
Meteors, 19, 125, 126, 145, 169,
175, 185, 186, 203, 227.
Meteor Crater, 193, 206, 208.
Meteorites see Meteors.
Montaigne, 34, 127.
Moulton, F. R., 222.
Nebular Hypothesis, 207, 221.
Neptune, 48.
Newcomb, S., 209.
Newall, H.F., 183.
Newton, Sir I., 10, 95, 98.
Newton, H. A., 145.
Nichols, 65.
Nuclei, see Comets.
Numbers of comets, 22.
Gibers, W., 52, 65, 127, 129.
Olivier, C. P., 73, 126, 146,
179, 187.
Orbits:
Changes, 25, 27, 169.
Elements, 233, 234.
History, 95.
Tail particles, 68.
Types, 20, 192.
Orlov, S., 88, 121.
Palitzsch, 99.
Pare, A., 7.
Passage through tails, 72, 73,
117.
Perrine, C. D., 35, 158.
Perseid Meteors, 142, 148, 185.
Peters, 185.
Pingre, E. G., 102.
INDEX
245
Planetesinal Hypothesis, 222.
Platina, 108.
Pogson, N., 144.
Pons, 34, 51, 127, 168.
Pont6coulant, Comte de, 100,
102.
Pope Calixtus III, 107, 108.
Proctor, R. A., 211.
Prominences, 214, 216.
Ptolemy, 6.
Pythagoras, 4.
Radial velocity, 44, 181.
Resisting medium, 54, 101,
165, 230.
Rosa, 150.
Rosenberger, 101, 109.
Riimker, C. L., 52.
Russell, II. N., 25, 42, 47,
227.
Santini, G., 129, 130.
Satellites, 225, 226, 227, 228.
Saturn, 47, 99, 191, 211, 227.
Schiaparelli, G. 142, 185, 212.
Schmidt, J. F. J., 42.
Schwarzschild, K, 66, 72, 122.
Secchi, A., 61, 80, 81, 141.
Seneca, 3, 9, 34, 153.
Sevign, Madame de, 12.
Shakespeare, W., quotations
from, 5, 127.
Siberian Meteoric Fall, 198,
228.
Slipher, 171, 173.
Smith, F. W., 146.
Sodium see Spectra.
Solar System, 210, 220, 223,
229, 232, 233.
Spectra, 44, 79, 123, 157, 161,
172, 179, 181, 218, 228.
Spenser, quotation from,
58.
Statistics, 20, 22, 23, 24.
Steavenson, W. H., 171.
Stormer, 90.
Struve, W., 32, 109.
Suetonius, 3.
Sun, 20, 29, 49, 69, 87, 92, 214,
219, 221, 223.
Superstitions about comets,
2, 3, 5, 6, 7, 9, 13, 14, 94,
107.
Swift, 35.
Tebbutt, 45, 153.
Thiele, 170, 173.
Thollon, 82.
Thorne, 45.
Thulis, 51.
Transits, 40, 117.
Tycho Brahe, 8.
Uranus, 47, 100.
Van Biesbroeck, 33, 56, 170.
Venus, 101, 178.
Virgil, 149.
Vogel, 84.
Vsechsviatsky, 57, 217.
Warner, H. H., 35.
Webb, T. W., 155.
246 INDEX
Weiss, E., 143. Yamamoto, 177.
Winnecke, F. A. T., 168. Young, C. A., 41, 81, 84.
Wolf, 110.
Wright, W. H., 124, 178, 181. Zanstra, 93.
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